Issei has developed many relationships throughout the series, both within the Occult Research Club and the Three Factions. He has also attracted the attention of most of the female characters.
She 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 as a heir of Gremory or a Devil. She and Issei both shared their first kiss with each other 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 constantly wakes up sleeping next to her naked. For this reason, she says that she has no problem with letting Issei see her naked and even let him touch her breasts from time to time, which she constantly says she has no problem with. In Volume 8, Issei and Rias go through a ritual where they are tested to see if Issei is worthy of being Rias' new fiancé. They both pass and become engaged while Issei remains oblivious to the situation, only mentioning that he would like to marry Rias one day.
In Volume 10, Issei was given permission from Rias' 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. Issei also gains a tremendous amount of power when he sees her breasts or touches them, replenishing his depleted stamina and energy and helped him achieve Balance Breaker (Volume 5) and Trident (Volume 9). This, however, has resulted in her gaining 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.
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 little sister. Later on, their relationship strengthens and rather than seeing her as his little sister, Issei starts seeing her as a single girl and a woman, this happened due to the fact that both of them kissed each other at the end of Volume 6 in the light novels. After that event, she became more bold and daring in their relationship, as it is implied that she give him a good morning kiss almost every morning, became tearfully happy when their love prediction is favorable and declaring 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 and joy and regained hope.
When Akeno first met Issei, she thought of him as a cute kouhai that she could spoil and flirt with, 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.
Despite continually fighting with the other girls for his affection, she finally declares her love for him in Volume 7 after he helped her reconciling with her father. She loves him a great deal and isn't afraid to get into fights over him with the other girls as she does get jealous, though she is a lot more calm 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 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. She, along with everyone else, later regained hope after finding out that Issei's spirit was alive.
In volume 7, Akeno appeared in front of him naked and wanted to have sex to forget about her sad past and her father. However, Issei refused, knowing it will just hurt her more and put her clothes back on, hugging her to comfort her.
At first, Koneko disliked Issei due to his lecherous mind and antics, but started to like 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. She respected him for being determined and not giving up. 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 because 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.
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. 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 and was also happy to hear his spirit was alive.
Kiba is the Knight of the Gremory group and is the self-proclaimed best friend of Issei. 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 in the beginning of the series. 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.
Issei himself has occasionally expressed worries that Kiba might be interested in him, due to the latter not having a girlfriend despite his looks and some occasional dialog hinting towards it, this might hold water when Kiba is under the effect of Azazel's Gender-swapping ray gun, but is not very likely otherwise.
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 Vol. 3, although 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 but nevertheless, she still loved 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 is having trouble in the harem because if she thinks or does anything bad, she will "fall". In volume 16 it is revealed that Irina is working to find a way for her to make love with a devil without falling.
Xenovia and Issei met in Volume 3 and did not care much for each other. Issei initially disliked her due to her calling Asia a witch and wanting to kill her, but after the events of Volume 3, she apologizes greatly and becomes 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 as 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. 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 frequently tries to seduce Issei whenever she can because she wants to give birth to strong children. She also emits a scary aura whenever the other girls are getting closer to him.
She does not get jealous when girls are around him, often at times just letting it go, although when one of the girls tries to have sex with him, she becomes jealous and tries to take him away. Despite having difficulties understanding her personality, Issei is willing to protect her just as much as the other 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 stuffs 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 appeance 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.
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.
Issei respects and admire 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.
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.
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 feeling's 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 feeling's, 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 help Issei learn how to achieve Balance Breaker and control the Trianna. 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.
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 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.
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 Oppai Dragon and her own magical girl television series, but is shown to get along well with Issei.
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.
Lord 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 is willing to accept him as a member of the Gremory, 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 this [Shalba Beelzebub and 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.
She is the mother of Ruval, Ravel, and Riser. She met Issei in Volume 10, initially Issei 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 Phoenix, after Lady Phoenix asking 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. 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. Issei didn't really hate him after he took back Rias and they 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.
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.
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.
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 good 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 women he loves.
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".
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, warning 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. Koneko vehemently protests by protecting Issei, saying that Issei is hers. 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.
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 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 publically ascociated with Issei. However, LeFay was able 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 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, LeFay has become a legitimate magician to hire.
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.
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 recognises 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 Yakasa. Cao Cao also defeated Issei fairly easily during their insuing battle and caused him to awaken his Illegal Move Trident 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.
Euclid is Grayfia's younger brother as well as Rizervim'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 appauled 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 armour. 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.
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, 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.
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. However, she is also oblivious to the standards of society and most emotion, as she would blatantly lift her skirt up just to prove the fact she did not wear underwear.
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.
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 (somewhat) proud and very worried about their son. Issei's mother even went on about how she was worried that Matsuda and Motohama were a very bad lecherous influence on Issei.
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 newfound popularity with the ladies and take great joy in starting bad rumors about Issei.
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 "Dragon Ball" (Drag So-Ball in the Anime) and ended up establishing a strange albeit friendly relationship.
Despite Issei's continual apprehensiveness around Mil-tan, he considers him to be a nice person and they appear to have a good relationship.
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.
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, she fell in love with him.
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.