Rias is the woman who reincarnated Issei and the woman he loves. When she first summoned him, she treated him as a "special" case, knowing that he had a lot of potential. As time went on, she later started to fall in love with him and her love became deeper after he saved her from Riser and for recognizing her as an individual rather than an heir of Gremory or a Devil. She gave her first kiss to Issei following the latter's duel with Riser. She often gets jealous when other girls flirt with or tempt him, even saying that Issei belongs only to her. She loves him very much and is willing to do anything with him. In response, Issei will do anything to protect her, as he loves her just as much as she loves him.
To add to that, Issei usually sleeps next to her naked. For this reason, she says that she has no problem with letting Issei see her naked and even lets him touch her breasts from time to time. In Volume 8, Issei and Rias go through a ritual where they are tested to see if Issei is worthy of being a candidate for Rias' future husband. They both passed and although Issei remained oblivious to the situation, he had mentioned that he would like to marry Rias one day.
In Volume 10, Issei was given permission from Rias's mother to start a harem to keep the girls around him happy. During this time, Issei was struggling to overcome the trauma that was caused by Raynare. This trauma is the reason why he could not believe that the women around him would actually be attracted to him. This causes tension to form between Rias and Issei until he proclaims his love for her in front of the audience during the Rating Game against Sairaorg Bael.
She was completely heartbroken by the news of Issei's death (which Sairaorg berates her for), but gained a great deal of hope when she found out that his spirit was still alive through his Evil Pieces. When she gains her derogatory nicknames such as "Switch Princess" (Bikou) officially used in Issei's "Oppai Dragon", an underworld's children's television show and "Crimson Bust Princess" (Azazel), Even though she hates those nicknames, she doesn't mind using her breasts for Issei's power ups, as she wants to be the source of his power and doesn't care if he is the Oppai Dragon.
In Volume 17, their relationship has improved to the extent that despite going out on a date with someone else, Rias chose to believe in Issei and the fact that he will still love her. At the end of the volume, Issei asks Rias if he could talk to her casually in private times, to which she happily replies yes.
In Volume 22, the two become engaged when Issei proposes to Rias after her graduation ceremony.
Issei is very protective of Asia ever since he met her, pitying the girl's suffering and was greatly saddened when she was killed by Raynare. Issei views Asia as a little sister that he needs to protect and is willing to fight anyone that tries to hurt her such as Raynare, Freed Sellzen and Diodora Astaroth. In fact, when Asia was supposedly killed by Shalba Beelzebub, the negative emotions of losing Asia made Issei enter the Juggernaut Drive and brutally mutilate him, nearly killing him.
Issei often has lecherous thoughts about her, but tries to block them, believing that he must protect her from everyone, including himself, and tries to think of her as a younger sister. At the end of Volume 6 in the light novels, Asia kisses Issei, causing him to see her more as a woman than his younger sister. After that event, she became more bold and daring in their relationship, as it is implied that she gives him a good morning kiss almost every morning. She loves him greatly, as she became tearfully happy when their love prediction was favorable and declared that she wants to bear his child.
She became heartbroken and devastated when she thought that Issei had died, spending most of her time crying and planning to commit suicide to be with him, though she knew Issei would be upset if she did so, confused for not knowing what to do. When she learned that his spirit is alive, she cried loudly out of relief, joy and regained hope, embracing him when he returned. Asia desires to stay with Issei forever, even if it means becoming one of his peerage, which happens in Volume 22 when she became his official Bishop.
In Volume 23, Issei proposes to Asia, promising to make her happy and be by her side for eternity.
When Akeno first met Issei, she thought of him as a cute kouhai with whom she could spoil and flirt, but as time went on, she started to see that Issei was very kind and caring, eventually falling in love with him in Volume 4. Despite Issei's hatred toward Fallen Angels, Issei never hated Akeno, seeing her as his kind senpai.
She would often make advances towards him, usually being more provocative than the other girls. In Volume 7, she tells Issei that she loves him after he helped her reconcile with her father. She isn't afraid to get into fights over him with the other girls as she loves him just as much as they do. She does get jealous when other girls get closer to him, though she is a lot calmer about it than Rias and the others. She is fine with the belief that she is third in his harem after Rias and Asia, as she likes the idea of an affair.
She was also so heartbroken when she thought that Issei had died that she disconnected herself from reality, remembering only the times that she spent with Issei, and cried terribly with her father as he comforted her. She, along with everyone else, later regained hope after finding out that Issei's spirit was alive, and she embraced him when he returned, begging him to never leave her again. She is psychologically dependent on Issei, as shown by her behavior after believing he was dead, and her display of anger when he lost in the Rating Game against Sona Sitri. Akeno gave an overwhelming killing intent towards Siegfried, who was a member of the group responsible for Issei's death, during their fated encounter on Ajuka Beelzebub's research center's rooftop.
She loses her "Onee-sama" aura around him sometimes and will act like a normal girl her age, such as when they went on a date in Volume 7. She was depressed when Issei had to pretend to be the boyfriend of Rossweisse in front of the latter's grandmother in Volume 17, but soon cheered up when Rias told Issei that he would have to go on a date with her and Akeno after his ordeal with Rossweise. Akeno has stated that she would love to bear Issei's children during the visit of Millicas in the human world. Issei also knows that she is emotionally fragile, so he cares for her a great deal and has promised her that he will not die.
In Volume 22, Issei both confessed his love for her and proposed to her through the broadcast of the Rating Game between his team and Baraqiel's team, during his final fight with Baraqiel. After the events, Akeno has constantly been calling Issei "Darling" and started behaving like she is his wife, despite being only engaged to one another.
In Volume 23, Akeno would take even more bold actions such as calling Issei "Husband" or "Dear" and wash every part of his body when bathing together.
At first, Koneko disliked Issei due to his lecherous mind and antics, but respected him after seeing his determination and tenacity for never giving up and also because of his willingness to go to great lengths to help a friend in need. Even though his methods got lecherous, she drew on his courage to help herself get stronger and braver.
She started to develop feelings for him after he helped her overcome her fear of Senjutsu, as Issei told her he was not afraid of her powers and would stop her if she ever went out of control. After that, she began sitting nowhere else but on Issei's lap, and began seducing him like all the other girls in the harem. She often gets jealous like the others if another girl is displaying any sort of affection or acting intimate with him. Still, whenever Issei says something that implies he is not a lolicon, she gets mad at him.
After Issei went into Juggernaut Drive (and after his return in Volume 12), Koneko has been having Senjutsu healing sessions with him in order to restore his life energy that was lost. During one session, she tells him that she would hate it if he died and that she would keep doing these sessions for the rest of her life if that was what it would take, making him promise to never use that form again. She offers a faster method to heal him, though it would involve them having sex, and a flustered Issei rejects the offer, as he wanted Koneko to be with someone she loves (as he still believed the girls around him didn't like him during this time).
Her love kept growing and growing until it reached its climax in Volume 11, where she went into mating season and chose him as her partner, but was stopped by her sister, Kuroka. Koneko soon after stops Kuroka's attempt to seduce Issei by declaring that he belongs to her. Later in the same volume, she proposed to Issei to be his bride, which Issei accepts under the condition that she grows bigger in both height and bust. She wants to be more than just a girlfriend to Issei because she chose him to be her soulmate. Her relationship with him is very serious, but will have to wait to develop more because her body is small and it would be dangerous for her to bear children at this point. When Issei died, she cried along with Ravel, but was also happy to hear his spirit was alive, embracing him when he returned.
Kiba is the Knight of the Gremory group and is the self-proclaimed best friend of Issei. In the beginning of the series, Issei is extremely jealous of Kiba due to him getting all the girls' attention with his good looks and had strong feelings of animosity towards him. Eventually, Issei started to think of Kiba as a good friend after spending time together with him whilst training and talking about their futures together. They both want to become strong so they can protect the girls of the Gremory group. Issei and Kiba have a good friendship and are willing to protect each other from harm. When Issei died, Kiba felt sadness and anger but kept it in. He released his anger on Siegfried, although losing but gained strength when one of Issei's Evil Pieces turned into Ascalon for him to use. Like the others, he gained hope when he found out Issei's spirit was alive.
Kiba also views Issei as a powerful rival and has several times stated he wants to be able to fight on his level or even defeat him.
Irina and Issei have been friends since childhood and reunited in Volume 3. Issei was surprised to see her because as kids, he thought she was a boy. She had a crush on Issei since they were little, but was shocked to see that he has become a Devil and a pervert. Nonetheless, she kept her feelings for him. She wants to make love to Issei and have his children, like Xenovia, to apparently strengthen Heaven's forces out of influence of Xenovia's naivety. She loves Issei very much, but had trouble expressing it like the other girls because if she thinks or does something impure, she will become a Fallen Angel. In Volume 16, it is revealed that Irina is working to find a way for her to make love with a devil without falling. In Volume 18, it is revealed that Michael has crafted a special room that is located in a different dimension that would allow her to copulate with Issei without fear of falling. It is also revealed that she was Issei's first kiss while they were children.
After the events of Volume 18 where Irina kissed him again, she began to frequently call Issei "darling" and became more clingy towards him, even learning to cook for him, bathe with him, and sleep with Issei more often. She becomes slightly less concerned about falling in order to be more intimate with him, even offering to have sex with him.
In Volume 23, during the match, Irina made a proposal to Issei which he accepts. Happy with his response and becoming engaged to him, Irina gained a massive power boost using the holy aura of Hauteclere to send Angels flying left and right. Irina wishes to have two children with Issei: one boy and one girl.
Xenovia and Issei met in Volume 3 and did not care much for each other. Issei initially disliked her, as she called Asia a witch and wanted to kill her, but after the events of Volume 3 apologized greatly and became a Devil after learning of God's death. She then decides that she wants to have strong children and chooses Issei to be her mate. She is completely oblivious to sexual situations due to her being raised in the Church and tries to have sex anywhere with Issei, even going so far as to bring condoms to school so they could "practice".
She wants his genes as the Red Dragon Emperor since she wishes to give birth to strong children as a way of making up time she feels she lost as a woman while in the service of God, rather than out of any romantic feelings she has for him. That changes after she begins to fall in love with Issei when he expressed his opinions to Michael to provide convenience for her and Asia to be able pray to God without experiencing the pain that came along with it. As the series progresses, it is shown that Xenovia has developed genuine romantic feelings for Issei due to his determination to protect his comrades even if he gets injured by doing so. She also emits a scary aura whenever the other girls are getting closer to him.
Xenovia has displayed concerns that the path to being with Issei is becoming farther as other girls get closer, causing her to be more aggressive in seducing him, shown in Volume 16. Despite having difficulties understanding her personality, Issei is willing to protect her just as much as the other girls. Like Asia, she also wants to join his peerage once he becomes a High-Class Devil, which happens in Volume 22 when she officially becomes his first Knight.
In Volume 23, Xenovia made a proposal to have him take her as his bride, which Issei accepts. Happy with his response and getting engaged to him, Xenovia gained a massive power boost. During the match, Xenovia wishes to have five children with Issei: three boys and two girls.
When Gasper and Issei first met, Issei thought that he was another girl and was shocked after learning that he was a cross-dressing boy. Issei often cursed him for being a shut-in, although Issei and Gasper became great friends and Issei trains him as his junior. Issei's blood gives Gasper a boost in power, even making him able to hit his Sacred Gear's Balance Breaker for a short time. After Issei apparently died in Volume 11, he seemed to transform in Volume 12, becoming filled with rage at the Khaos Brigade for killing his friend and senpai, and proceeded to defeat Georg with his new power.
The two of them are a proper senpai-kouhai, and Issei has taught Gasper several things and also helps Gasper with his troubles. The two are on extremely good terms and are a strong team. Issei appears to also teach much of his perverted ideals to Gasper, which the latter seems to be innocent enough to admire. Twice, Gasper has even helped Issei to use his dress break by freezing their targets in place beforehand with his Sacred Gear.
Rossweisse is the second Rook of Rias' peerage and a teacher at their school. Issei and Rossweisse have a student-teacher relationship, where Rossweisse is shown to be giving advice to Issei on various stuff from Issei's studies in school, training, and even his relationships with the girls. Rossweisse later gained some interest in Issei during Volume 12, where she and the Occult Research Club members all asked Issei out on a date.
In Volume 17, she asks Issei to become her boyfriend due to her grandmother coming to visit, to whom she told that she does have a boyfriend when she actually doesn't. Later on in the volume, they go out on a date to Tokyo which was interrupted by the appearance of Euclid Lucifuge. During the attack on Auros and Agreas, Issei rescues Rossweisse from Euclid, defeating him in the process. By the end of the volume, Rossweisse develops feelings for Issei, even asking him out on a date to a 200 yen shop next time. In Volume 22, under Rias' orders, Rossweisse was transferred to Issei's peerage, becoming his first official Rook.
Ravel and Issei met during the Rating Game between Rias and her brother, Riser, and didn't really care for each other. After he defeated Riser at the engagement party, she became infatuated with Issei and starts to develop feelings for Issei, albeit in a tsundere fashion. In Volume 10, she moves in with him and transfers to his school in order to spend more time with him. She has stated that she does not mind becoming one of his concubines as long as she is by his side. Issei, however, remains oblivious to her feelings but was still willing to put his life on the line to protect her under Lady Phenex's request to take care of her. It is implied in Volume 9 that Ravel enrolls in Kuoh Academy so she can get closer to Issei, and moves in with him in the following volume. She happily accepts Sirzechs's request of becoming Issei's manager and continually works hard for him, such as finding a suitable magician for him to make a pact with.
Issei respects and admires her a lot for the knowledge and dedication she puts in her work as a manager and completely trusts in her judgement. He has stated that he would like to have her as his manager forever and maybe as a tactician in his peerage when he becomes a High Class Devil with his own evil pieces. Issei officially becomes a High Class Devil in Volume 22, when Ravel officially became his Bishop, along with Asia.
Sona at first treated Issei like a regular servant, but as time went on, she grew to admire him for his tenacity and courage. She respects him greatly for everything he has done, but she is also jealous as well because Issei solved many of Rias' problems for her which she, as her best friend, could not do anything about due to her position as a High-Class Devil. She considers him as a friend as he is Rias' boyfriend and suggested to Issei that he should call her by her first name, and told him to take care of Rias even though she's stubborn and has a great deal of pride. She has admiration and respect for him and for his accomplishments and everything he has done for Rias, and she also congratulated him on his promotion recommendation to Middle-Class Devil.
Sona's Queen, of whom Issei is aware of her feelings for Yuuto Kiba and often encourages her to try her best to get his attention (mostly worried that Kiba might be interested in him instead). Issei and Tsubaki also exchange emails regarding Kiba, much to his confusion.
Saji and Issei have a friendly rivalry going on because they are both Pawns, they both harbor feelings for their masters, and they both have Dragons sealed inside their Sacred Gears. At first they did not care for one another, but as time progresses, they eventually became good friends, even stating that they are "War Buddies" in Volume 3 of the light novels and have started training to become strong to face one another again. When Issei died, Saji showed expressions of anger and sadness due to him being killed and wanted to avenge Issei.
She congratulated Issei for his promotion recommendation to Middle-Class Devil and asked for his autograph, saying that Issei is the type of guy that she likes because he is always muddy after a fight, and admired Issei for his fight with Sairaorg Bael. She has a crush on Issei.
Issei first met Bennia in Volume 14, as she begged Sona to bring her along to Issei's home since she is a huge fan of his, being an Oppai Dragon enthusiast and having a large collection of the Oppai Dragon goods which she puts on the back of her robes.
Not much is known about Issei and Momo's relationship, but when it appeared how he was becoming close friends with Sona, Momo cheered him on to get Sona to fall for him so she could date Saji.
Not much is known about Issei and Ruruko's relationship, but like Momo, Ruruko gives him support to get Sona to fall in love with him so she could also date Saji.
Raynare was Issei's first girlfriend, who disguised herself as Yuuma Amano to trick and deceive Issei in order to kill him because he was deemed as a "threat". She killed Issei with a light spear at the end of their date, and afterwards killed Asia by stealing her Sacred Gear, Twilight Healing. This caused Issei to hate Fallen Angels altogether. Issei later defeats Raynare and she again tried to deceive him (due to her becoming Yuuma once more) when she was trying to convince him to spare her life, but he sees through her ruse and asks Rias to kill her.
Although dead, Raynare left a permanent mark in Issei's heart, as explained in Volume 10 when Koneko, Asia, and Akeno try to "heal" Issei. She made Issei fear women after he planned his whole first date with Raynare, only to be killed and treated terribly by her. She also made him afraid to call girls by their first name (i.e. calling Rias "President (Buchou)" instead of her name), and also made him afraid of being disliked by other girls or rejected if he ever showed his feelings again. After being laughed at by Raynare in their final confrontation, Issei also seemed to form a sort of mental block, making him oblivious to the feelings of the women around him. For example, before Volume 10, he constantly assumed that Akeno and Rias's attention is because he is a 'cute kouhai' and something like a pet to them. The girls, however, eventually figure this out and Raynare's "curse" is lifted from Issei's heart. After this, he is noticeably more perceptive of the girls feelings, though he is still oblivious at times.
Issei, at first, hated Azazel because he was a Fallen Angel, but learned later on that he means well and has good intentions. Issei respects Azazel a lot, even calling him "Sensei" after Azazel tells him that he would teach Issei the secrets of the Sacred Gear and how to ensnare beautiful women. Azazel later became the Occult Research Club's advisor in Volume 4, and aided the Gremory team on how to better improve themselves as a team. He also helped out in progressing Issei and Kiba's Sacred Gears, as well as helped Issei learn how to achieve Balance Breaker and control the Triaina. He also shows a great deal of concern for Issei, as shown after Issei's death, but was relieved to hear his spirit was alive. They seem to be very close to each other as they are usually seen discussing various matters regarding the mythical events and the people around him. Issei is very close to Azazel to the point that in Volume 22, he is a bit saddened knowing the fact that Azazel is not present to his High Class Devil promotion and wanting Azazel to see his achievements, because of his great sacrifice of fighting Trihexa at the isolation barrier.
Baraqiel at first hated Issei because he misunderstood the meaning of Oppai Dragon as a Dragon who feeds on breasts, instead of a Dragon who loves them and believed that Issei wanted to eat the breasts of his estranged daughter, Akeno. However, since coming to understand the mistake and having been reconciled with his daughter because of Issei and Akeno's feelings for him, Baraqiel has shown his approval and respect for him.
Issei sees Michael as a very powerful leader and respects him very much. Likewise, Michael has also shown to have a great liking towards Issei. In Volume 4, Michael gave Issei Ascalon as a symbol of peace between the Three Factions prior to the events of the Kuoh Treaty. Also, Michael agreed on Issei's request of allowing Asia and Xenovia to pray without receiving damage as a devil. Later on, he, along with some of the angel researchers, developed a room to allow angels to procreate more easily in support of Issei and Irina's relationship.
There is not much to say about their relationship, other than the fact that even in their first meeting, Issei always thought lecherous thoughts of her, but at the same time, acknowledged her position as one of the Seraphs and her beauty as the most beautiful woman in Heaven. in Volume 18, She came to greet Michael and the Gremory team while also wearing a Santa costume. While Issei was thinking lewd thoughts of Gabriel, she hit his forehead in an adorable way. Gabriel returned to Heaven from a business trip in the Human world after hearing about the attack in Heaven. She was unaware that her breast was seen by Issei due to him using his newest ability "Penetrate".
They seem to have a friendly relationship and respect one another. Dulio sees him as a brave Oppai Dragon hero, like Issei sees and respects Dulio as a suited powerful leader of the D×D. Before their match in the Azazel Cup, Dulio playfully told Issei that Mirana has a large bust which will useful to him. During the match, Issei and Dulio displayed a friendly rivalry, as they wished to defeat each other.
They have seen to have a friendly relationship with each other, as it is seen when she entrusted Xenovia in Issei`s care, claiming that she will live a happy life with him and her friends.
Sirzechs is Rias' older brother and met Issei during his duel against Riser at Riser's and Rias's engagement party. He shows a great interest in Issei and considers him part of his family, even asking Issei to call him his big brother, though Issei considers it impossible to call a Satan brother. He showed anger at Issei's death and, like the others, wanted to avenge him. He was also happy to hear that his spirit was still alive. He has shown great faith in Issei's relationship with Rias as they progressed on. He later passed on his role of "taking pictures of Rias while she was sleeping" to Issei, though Rias was upset and said she didn't mind Issei doing it but heavily rejected Sirzechs with doing such things.
She has a minor rivalry between Issei's show Oppai Dragon and her own magical girl television series, but is shown to get along well with Issei.
Ajuka appears to like Issei, even being willing to correct the overwritten code of his Evil Pieces because of the Juggernaut Drive to prevent malfunctions and along with this intentionally approving of cheat-like abilities in Rating Games. Whether this is more of an experiment on his part, a favor to Sirzechs or for some other reason is still unknown.
Falbium is the laziest among the four Satans to the point that he leaves everything to his servants much to Issei's dismay but he still helped in administering a trial for Issei and Rias in Volume 8 dressing as Satan Green.
Zeoticus Gremory is Rias' father and meets Issei through her (although after the fight with Riser). He at first was disappointed at the fact Rias did not marry Riser to connect the families, but later accepted his daughter's content of loving Issei. He became fond of Issei and becomes good friends with Issei's parents and accepted him happily into his family, even wanting Issei to call him father. Like Sirzechs, he also shows great faith in Rias and Issei's relationship, although he often embarrasses the two (more often Rias).
Venelana is the mother of Rias. She, like her husband, was also very disappointed at the fact that the engagement between Riser and Rias broke off. She, however, decides to accept her daughter's decision to love Issei and became fond of him. She is willing to accept him as a member of the Gremory family, even wishing for him to call her Hahaue (respectable way of saying mother) or Okaa-sama (mother). She teaches Issei how to dance and eat elegantly and become a proper gentleman so that he does not embarrass Rias at social events in the Underworld. Issei tends to have lewd thoughts about her as she has great resemblance to Rias.
Grayfia is the wife of Sirzechs and Rias' sister-in-law. She first met Issei while retrieving Rias, who was in the middle of trying to make love with him. She was there after the Occult Research Club's defeat in the Rating Game against Riser, and helped Issei bring Rias back and stop the engagement. She shows great concern for Rias but accepts Issei as her lover. Like the rest of the family, she is also fond of Issei. In Volume 12, she was one of the few to not even consider the possibility of his death, stating that she 'would not allow the person who will become her brother-in-law to perish with something like Samael's curse'. At the onsen in Izu, it is revealed that she is very happy about Issei joining the family and treats him as her little brother and washes his back, much to his surprise. She also demands to be called "Ane" (elder sister) by Issei— much like how Sirzechs wishes to be called "Onii" (elder brother).
Like his father, Millicas likes and respects Issei calling him "Issei nii-sama". In fact, when Sirzechs asks Millicas whether he likes Oppai Dragon or Satan Red more , Millicas chose Oppai Dragon much to the shock of his father. They have a brotherly bond, he even asks Issei to call him by his name without honorifics due to feeling lonely about Issei calling Rias by her name which made him very happy when Issei did.
She is the mother of Ruval, Ravel, and Riser. She met Issei in Volume 10, Issei initially thought that she would hold a grudge against him for his action of destroying the engagement ceremony, but to Issei's fortune there are people with big hearts in the house of Phenex. Lady Phenex asked him to take care of Ravel so no weird pests (boys) would come near her, knowing full well that Ravel is in love with Issei and was willing to only be of use to him and be one of his concubines.
Issei initially disliked Riser because he tried stealing Rias away from the group, which Issei did not want. At the end of the Rating Game between Riser and Rias' peerages to determine whether Rias should wed Riser, Issei went against Riser but ran out of stamina and was beaten to near-death before being saved by Rias, who surrendered in order to prevent any further harm to him. He woke up two days later and went after her, interrupting the engagement party, and Sirzechs decided on a one-on-one battle. During that fight, Issei gave up his left arm to reach his Balance Breaker for a very limited amount of time consisting of ten seconds and ended up defeating Riser with a strategy/fallback plan that should his armor disappear before he defeated Riser. Issei made him go into a state of depression due to the fact that he has never lost and also made him afraid of Dragons. Issei didn't really hate Riser after he took back Rias, as shown when he agreed to help him get back on his feet under Ravels request, at that time the two developed a friendly rivalry and even talk with each other via magic circle, but Riser is still jealous of Issei's relationship with Rias and wants to fight him again. Later on during the invasion of the Underworld, after he overcame his fear of Dragons during Tannin's training which was organized by Issei and Ravel, he went out fighting in an attempt to counteract the monsters and also tried to help Ravel deal with Issei's death.
One of Riser's Rooks, after Issei defeated her in the Rating Game between their masters with Dress Break and Dragon Shot, Isabela started to respect Issei's strength, even stating that if he becomes even stronger she'll be able to boast about her story as well.
One of Riser's Pawns, she initially disliked Issei for being a pervert and using Dress Break on her during their masters Rating Game. However in DX.1, she apparently becomes a fan of his, even asks for his autograph.
Sairaorg is the cousin of Rias who met Issei when they were deciding Rating Game matches between the young Devils. He is known as the strongest young Devil. Wanting to fight each other from the beginning, they then sparred before their match, in which Issei found him to be extremely powerful and Sairaorg finding Issei to be interesting. Issei held incredible respect and admiration for Sairaorg for his strength and relentless determination, addressing Sairaorg with the honorific-san
During the Rating Game match between Rias and Sairaorg's peerages, Issei and Sairaorg went head-to-head in a fist-fight match. Later, Issei found that Regulus, his Pawn, was a Sacred Gear which he combined with and activated his Balance Breaker. The fight continued and ended with Sairaorg being knocked out while still standing. After the match, the two became close friends and promised to get stronger to fight each other once again later on. When Issei apparently died, he came and encouraged Rias to get up from her depression. He believed that he was not dead due to the fact that he had not had sex with the woman he loves.
Despite having never personally met each other, according to Zekram, Sairaorg's father holds a strong hatred towards Issei, as Lord Bael is very jealous of Issei's power, fame and achievements as the strongest young Devil, despite being a reincarnated Devil; a fact which strongly opposes his personal viewpoint of Devil blood purity.
Issei hated Diodora from the start as he was trying to take Asia away. Diodora, in turn, looked down on Issei, referring him as a "filthy Dragon" and stated that being touched by him made him feel sick. Issei's hatred for Diodora grew to its peak when he learned that the noble set up Asia on purpose on the day she healed him, which resulted in her excommunication from the Church and labeled as a heretic, just so he could add her to his "collection".
Issei at first admired and respected Belial as he was at the top of the rating games. At volume 20, he was saddened when he learned that Diehauser was in fact a traitor, and tried desperately to stop him from revealing the corruption of the rating games as it would cause chaos and unrest. When Diehauser revealed his true intentions, Issei regained his respect for him.
At first the two of them have a vague relationship, as Issei tries to avoid her as much as possible because of her cold and strict nature. They later meet up, and the two of them start to get along, knowing the fact that both of them are Otaku fans. She later gained an interest in Issei through their similar compabilty in Otaku mecha, shorty there after they are on friendly terms.
They first meet at the Gremory training area in Volume 23, where Roygun has become Issei's advisor for the Azazel Cup. She took a interest in Issei, flirting with him and calling him "Cute", which leads Issei to become embarrassed. Roygun ask if Issei doesn’t like older women as she's over a century years old, which Issei responds that sees her a beautiful onee-san in her early twenties, much to Roygun’s happiness. Roygun found Issei’s words comforting and was happy that he complimented her. After this, she tells Issei she will be watching his match against Dulio. After the match, she reappeared before Issei again and joins his team, while at the same time, pledging her and her peerage's service under him.
Vali is Issei's rival and the holder of the Longinus, Divine Dividing. When they first met, Vali was on another level compared to Issei, which made Issei devoted to becoming more powerful. During the signing of the peace treaty between the three factions, he went against everyone and formed a team in the Khaos Brigade. This made Issei hate him and after which they fought with Issei in Balance Breaker mode due to Azazel's bracelet. Issei won the fight due to Vali teleporting away. Later on, however, the two joined forces to defeat Loki, and after that, began cooperating. Even later into the series, the two started to become friends, although still in a rivalry. They promised to fight each other someday and to become even stronger.
Kuroka is the elder sister of Koneko. She met Issei in Volume 5 when she tried to take Koneko, with Issei and Rias stopping her. He fought her for Koneko, and almost punched her in the face, but stopped at the last moment and warned her that if she ever tried anything again, his fist wouldn't stop. During Koneko's mating season, she made matters a little worse by stimulating Koneko and Issei. Although she stopped her sister, she still began to seduce Issei because she wants his "genes" for strong babies, which she tried with Vali but he denied, though it was stopped by Koneko. Issei later learns more about her past while she is receiving healing and tells Koneko that she is a good person, although she remains a "naughty cat". She has an attraction towards Issei and later develops strong romantic feelings for him.
Le Fay is the witch of the Vali Team whom Issei meets in Volume 9, where it is revealed that she is a huge fan of his show, Breast Dragon Emperor Oppai Dragon (Oppai Dragon for short), wanting to shake hands with Issei and asked for an autograph upon meeting him. She later gives him advice on boosting his magic power as well as choosing a magician to form a contract with. She herself was considered a perfect candidate to be that magician by Ravel's statistics, but unfortunately Ravel refused to let her take up that position, because of Le Fay's status as a terrorist of the Vali Team rendered it impossible for her to become publicly associated with Issei. However, LeFay was able to become Issei's contracted magician, because it was revealed in Volume 16 that because the Vali Team had been pardoned of their crimes and that Arthur had requested Issei to take her as his contracted magician for the reason of protecting her. Issei accepted his request, and asked Ravel, who agreed since she said that since the group has been pardoned of their crimes, Le Fay has become a legitimate magician to hire. Le Fay appears to have some affections towards Issei, expressing desire to go a shopping date with him.
Arthur seems to have deep trust towards Issei by leaving his dear sister, Le Fay, in his care, believing that he is able to protect her and have her return home to her everyday life by having her as his contracted magician. Issei accepted his request and Arthur intends on repaying him for it.
They first meet each other in Volume 15, alongside Dulio and sister Griselda. There he and Dulio help Issei and Saji have a sparring and train briefly the two dragon possesors, they seen getting along to each other and have a friendly relationship as he usially helps Issei and the Gremory group from behind the scenes during the sudden attack of Walburga. They also make a friendly chat about Vali’s history and his arrogance in childhood.
They have seem have a friendly terms to each other. Odin acknowledges Issei's strength and power, also he sees Odin as very powerful leader of the Norse Faction.
Indra and Issei do not really have interactions to each other. But Indra seems to acknowledge his power and strength, but at the same time Sakra will not hesitate to eliminate Issei alongside his soul if he will be a threat to the world. Which is why Azazel protects Issei from Sakra from their inside matter. Indra considered Issei as a clown for his heroic efforts as Oppai Dragon despite being a Devil.
Like Azazel, Issei seems to dislikes Hades ever since he met him in the beginning. He hates him knowing the fact that Hades despises Devils and Fallen Angels and for secretly helping the Khaos Brigade from behind the scenes.
They met in Volume 22 where he greets and congratulates Issei for his promotion to a high-class devil. He also revealed and read Issei's real thoughts and where Ophis was at. He also asked Issei to join his side to fight Indra in the future, much to Issei's shock. Though he didn't answer Shiva directly, the two of them seem to have gotten along well and Shiva promised Issei that the two of them will have a good relationship and that he will protect him from behind.
Issei met him in Volume 22 alongside Shiva to personally congratulate him, also asking him to help them fight Indra in the future. The two of them seem to have gotten along well and Mahabali seemed to have an interest in fighting along side Issei or having a duel with him in the near future.
He first met Sun Wukong during his intervention with the Hero Faction, allowing him to overpower several members of the Hero Faction including their leaders. He assisted Issei using his Senjutsu to badly injured Cao Cao and help him to bring back Yasaka's consciousness. They seen have a friendly and good relationships as he volunteered himself to be Vali and his mentor to be strong and have achieved that equal to a God class opponents.
Issei hated Shalba for his past action to Asia. Because of Shalba's growing insanity and his viewing Ophis as merely a tool to raise his own power, Issei's hatred towards him only grows stronger. When Shalba views the world has rejected him and tries to destroy the underworld and kill the children using Bandersnatch and Jabberwocky, Issei recognizes that he has basically gone insane and outright kills him to save Ophis and protect the children. This would make Shalba the first person Issei himself has ever actually killed. Ironically Shalba managed to use Samael's poison to finish Issei off as well but, unfortunately for Shalba, Issei's soul survived despite his body being destroyed and would later receive a new body.
Cao Cao and Issei start off with an initially hostile relationship due to the former being a leader in Khaos Brigade and for abducting Yasaka. Cao Cao also defeated Issei fairly easily during their ensuing battle and caused him to awaken his Illegal Move Triaina ability. Following their confrontation Issei did manage to rob Cao Cao of one eye, which would ultimately be replaced by an eye of Medusa. Issei's dislike of him is furthered when he attacks him and his friends, later to take Ophis' power away from her. Cao Cao himself, however, seems to hold Issei in fairly high regard during their later confrontations, especially after Issei returns from the dead, which even manages to make Cao Cao believe he can only be a true monster as he came back with Great Red which according to Cao Cao can't be referred to as luck. Issei's earlier wounding of Cao Cao's eye continued to be a factor even in their final battle, as Issei managed to use Samael's poison on his Medusa's eye and cause him near fatal levels of pain. In Volume 18, they have seen have no antagonistic relationships and supporting each other and promised that someday they will fight once and settle the score between them.
Euclid is Grayfia's younger brother as well as Rizevim's right hand man. Issei and Euclid's relationship was immediately hostile, as the latter is responsible for attempting to abduct the first year members of the Occult Research club and sending several wizards to attack them and finally even aiding in the revival of the Evil Dragons.
Furthermore, a little later on, both Issei and Ddraig developed a personal grudge against Euclid for creating a replica of the Boosted Gear from the remains of Issei's previous body and sacrificing the souls of famous dragons, an act even Vali seems to be appalled by. During their fight in Volume 16, Euclid was able to overpower Issei's Cardinal Crimson Promotion with his own Scale Mail armor in its normal Balance Breaker state, this is stated to be because Euclid's natural strength is equivalent to a Satan's even without the armor. Issei was able to turn the tables on Euclid only after using his newly awakened power and further theories that Euclid is observing him in order to learn how to use more of his techniques.
During their rematch in Volume 17, Issei was able to overpower Euclid by making the white wyverns into red that are able to use Ddraig powers which shocks him and the latter was defeated by Issei by unlocking Longinus Smasher thus defeating him and destroying his armor in process.
Upon their first meeting, Issei disliked Elmenhilde for her intention to take Gasper for his ability and was angered when she discriminated Issei for claiming him to be nothing more than a mere servant with no qualifications, despite being the Red Dragon Emperor. After the events that traumatized her in Volume 16 and reunited in Worldly Desires of Steel short story after he saved from a stray devil and touched by his kindness, her selfish demeanor has lowered and they seem to have developed a friendlier relationship, with Elmenhilde appearing to have also developed feelings for him. Eventually her feelings for Issei is what led her to join his team for the Azazel Cup.
She and Issei have shown to be on good terms with each other, as shown in volume 20 when she comforted Issei when he was being worried about Ravel.
Ddraig is the Dragon known as the Red Dragon Emperor that is inside Issei's Sacred Gear/Longinus, Boosted Gear. He gets along with Issei very well and Issei never thought of him as a tool, but later on when everyone starts calling him the Oppai Dragon, he becomes extremely depressed due to his credit being demolished little by little, going so far as to require medicine and counseling for the depression. He has a great interest in Issei due to him never giving up and protecting the ones he loves, also for being the possessor who talks to him the most and not seeing him as a tool. When he was with Issei when he died, he was devastated and told Ophis he was the greatest possessor of him, although he comes back along with Issei.
Tannin is the Dragon that Issei trained with in the Underworld. He was the one responsible for helping him become stronger and attain his Balance Breaker. Issei and Tannin have such a close relationship that Issei calls him "Old-man Tannin". The two are really good friends and have fought numerous battles with each other. He was, at one time, a Dragon King and knew Ddraig before he was sealed.
Ophis officially met Issei at his house when Azazel invited her over to talk. At first, he was afraid of her because he thought she would destroy them immediately. Although she has no intentions other than to defeat the Great Red and return to her birthplace in the Dimensional Gap. At first, she took a huge interest in Issei because of how Ddraig was and reacted with Issei and how his color had changed from red to crimson. In Volume 11, Issei became Ophis' first friend, and he invited her to live with him in the Hyoudou Residence. Ophis is also Issei's last friend (according to Ddraig) prior to his reincarnation as a humanoid Dragon. In Vol. 12, she plays a big role in reviving Issei with the Great Red's help, and now stays with Issei. In Vol. 20, Issei is shown to care deeply for Ophis as he became enraged when she was attacked by Níðhöggr and Ophis shown to care fondly for Issei, as Issei was her first friend and wants to help him. Thus helping Issei by lending him Ophis's own power which enables him to unlock Diabolos Dragon which is so great that it overwhelms Rizevim Livan Lucifer's Sacred Gear Canceler, previously thought to be impossible.
Ophis is similar to a child mimicking everything she sees, as she has often wanted to do things normally done by the other loving members of the Gremory group to lssei.
The relationship between Issei and the Great Red is somewhat vague, as Great Red rarely communicates with anyone and most of the communication between the two has been through Ddraig. However, Great Red was willing to save Issei and Ophis when the pocket dimension they were in collapsed, hurling them into the Dimensional Gap. He also crafted a new body for Issei from his own flesh.
Because Issei's present body has been made from the Great Red's flesh, Issei can merge with his power with no effort. Issei thinks of Great Red as almost like a super-powerful delinquent because of his tendency to just fly around the Dimensional Gap and taking offense at being glared at. Issei is also embarrassed by Great Red's apparent fondness of the Oppai Dragon theme song, a line of which is the only words directly exchanged between the two.
At first they were antagonistic to each other, but at the time of volume 20 he no longer possesses any bad thoughts that lingering towards Crom Cruach. As it's seen when Issei bows his head and apologize to him for accusing him in what happens to Ophis, but Crom Cruach says there's no need for a Heavenly Dragon to bow his head to an Evil Dragon like him.
Their relationships are somewhat vague, as they usually don't show any signs of antagonism to each other. Lilith seems to be interested in Issei, knowing that he possess Ddraig, as shown when she leaves the Qlippoth to see Issei again as well as another her (referring to Ophis) and Great Red. After moving into the Hyoudou Residence, Lilith became attached to Issei as she starts to sit on his lap, competing with Koneko and Ravel over it.
Their first meeting seems to be antagonistic, were Issei thinks that he is a very brutal evil dragon but this proven wrong when Apophis mentions that he disdain and distaste of brutality and barbaric acts to Evil Dragons like Grendel and Níðhöggr. After their battle Issei respects him due for Apophis having an honorable side although he done so many evil things.
Issei meets Bova in the Kuoh town's underground in Volume 22. Issei was shocked to learn he is Tannin's third son and the strongest amongst the children. Bova has shown greatly he respects Issei and the only one who hasn't given him a bad attitude. Bova wishes to stay by Issei's side as his subordinate rather than his servant. Eventually, Bova became a member of his team for the Azazel Cup. Bova even calls Issei "My lord".
Issei first meets Tiamat at the Occult Research Club when Ouryuu and Millarca introduced her in Volume 23. When Tiamat demand to Ddraig to give back her treasure he borrowed, Issei tells Ddraig to give it back to her, which Tiamat tells Issei should also take responsibility as well, as he is the current possessor of the Boosted Gear. After that Tiamat warns Ddraig that she will not hesitate to burn him and Issei if he runaway like he always did, much to Issei's shock. He eventually becomes her guarantor until he wins the tournament and wishes to bring her treasure back.
In Volume 9, he helped save Yasaka from the Hero faction, using her leyline to summoned Great Red, after the hero faction retreat, he then help Kunou to wake her mother`s consciousness using his bilingual, with Sun Wukong assistance. They met once again in volume 19, where she entrusted Kunou to him and attempted to seduce him to have children with her, by Kunou`s protested to have little brothers or sisters much to Kunou`s annoyance. They both have friendly relationships.
At first, she was under the impression that Issei kidnapped her mother and attacked him and his friends. After he saved her mother from the Hero Faction in Kyoto, Kunou developed a crush on Issei and told him she would always wait for him. She is also the young leader of the Youkai in Kyoto.
They are Issei's two closest human friends and fellow members of the Perverted Trio. The three will often get together and perform such activities as peeping and watching/reading porn. However, when push comes to shove, the two are completely willing to leave Issei to his own devices and receive their share of the punishment. The two have become very jealous of Issei's new found popularity with the ladies and take great joy in starting bad rumors about Issei and beat him up when enraged.
Despite his current rocky relationship with them, Issei does seem to care for them almost as much as anyone close to him, as he has stated that if possible, he would want to remain friends with them for the rest of his life.
Issei always complains about Kiriyuu that she always teaches/tells Asia erotic things. However, the two are on relatively good terms when not at odds, as she tends tease him and Matsuda and Motohama. In Volume 19 it is revealed that she is aware of the truth about Issei and his friends, but assures to keep it a secret from everyone, showing a certain degree of trust.
She met Issei when Rias needed to write a report about Youkai in Volume 8, but she didn't want to do it because she feared that she might lose her soul should she get involved with Devils. However, she agreed to do it if the Occult Research Club could prevail over her team in a tennis match, with the conditions being that if Rias lost, she would have to give up Issei temporarily. Unfortunately for her, she lost and had to write the report. Later on, she asked Issei to help her break an engagement that her father set up. Issei decided to help her in a battle of beasts against her father and in the process of helping her, fell in love with him.
They first meet in Volume 22, Ouryuu has been wanting to meet Issei for a long time and been asking his student council president Xenovia to meet him. When they meet, Ouryuu expressed his admiration towards Issei, despite him being a normal high school student and was put into many dangerous situation, Issei overcame it all and his possession of the Boosted Gear of the Welsh Dragon.
Under Tiamat's approval, Ouryuu joins Issei's team for the Azazel Cup as his "Fist of the Red Dragon Emperor of the Blazing Truth" and chooses to become his pawn after hearing that Issei originally began as pawn. Upon training together, Issei was impressed at Ouryuu's physical prowess and combat skills. After watching Issei comforts Ravel, Ouryuu respect and admiration for him grew to the point that he wishes to do better for him in future international rating games.
Despite the fact that Issei constantly complains about his parents, Issei does love them deeply. In Volume 4 when Vali mentioned that he should try killing Issei's parents, Issei was so mad that he wanted to kill Vali on the spot. His parents are like any normal parents, who are very proud of and love their son dearly. Issei's mother even went on about how she was worried that Matsuda and Motohama were a very bad lecherous influence on Issei. As of Volume 20, his parents found out about his secret life, in spite of that they still accepted their son as he is.
Irina's father that used to be Issei's neighbor who was like an uncle to him and seem to have a friendly relationship. Touji approves of his daughters feelings for Issei, in fact he encourages them to have children so he can be a Grandfather and offers his full support, which usually leaves the two embarrassed. Touji has complete trust in Issei that he is able to protect his daughter.
Issei's first interaction started off rocky with Morisawa doubting Issei's abilities as a Devil and preferring Koneko over him. However, they soon bonded over their childhood love of Drag So-Ball ( a parody on "Dragon Ball") and ended up establishing a strange albeit friendly relationship over popular anime and games.
Despite Issei's continual apprehensiveness around Mil-tan, he considers him to be a nice person and they appear to have a good relationship.
Elsha is the second strongest and the strongest female Boosted Gear possessor. The two meet each other in volume 9 where she and Belzard help both him and Ddraig to awaken his new found powers, and also supporting the two of them from the depths of Boosted Gear. They have seem good relationships to each other although they have meet for short period of time, Elsha and Belzard says their final words after they vanish while smiling at Issei.
The strongest Boosted Gear possessor that defeated two White Dragon Emperors in his lifetime. He alongside Elsha help Issei and Ddraig to awaken their new found powers. He sings the Oppai Dragon song while leaving with a smile along with Elsha.
At first all of them didn't have interest of supporting Issei and force him to go berserk in volume 10. But after Issei vanquished the curse and promised to them that he will show the future they have seem supporting Issei until volume 11 and 12 where they sacrifice themselves to save Issei's soul from Samael's curse.
Past Divine Dividing Possessor
It is revealed in volume 10 that a Divine Dividing possessor resides in the Boosted Gear when Issei use Albion's jewel where some of his thoughts resides. He helped Issei to reduced and halved the negative emotions and the curse of the Juggernaut Drive that covered the past Boosted Gear possessors. Though Issei doubt at first that he will help him he just smile, and said that he is an interesting guy and counting that he will lead both the Two Heavenly Dragons. He and the rest of the past seniors of Boosted Gear sacrifice themselves in order to save Issei from Samael's curse.