Rias has a lot of affection towards Issei from the start of the series, which seemed to have grown as the series progressed. This is highly attributed to the Gremory's tendency to have close relations with their servants as compared to other clans. Rias doesn't seem to mind allowing Issei to gaze on her naked body. Following the events of Volume 2, Rias falls madly in love with Issei and attempts to keep him away from the other girls who may have affections for him. This includes, but is not limited to, Akeno, Asia, Xenovia, Koneko, Irina, Ravel and Rossweisse.
She loves him very much and is willing to do anything with and for him. In response, Issei will do anything to protect her, as he loves her just as much as she loves him. After becoming Issei's girlfriend, Rias begins to show more confidence in her relationship with Issei, no longer being easily jealous or angered by Issei being close to other girls like she previously does, even accepting Issei's dream of having a harem. She was completely heartbroken by the news of Issei's death and locked herself in her room, crying (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. Despite hating being called the Switch Princess, Rias seems to only enjoy the title when she gives energy to Issei, wanting to be the source of his strength. 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, although Issei remained oblivious to the situation.
In Volume 22, Rias was proposed to by Issei to which she tearfully agreed and the two become engaged.
Akeno is Rias' first servant, being her Queen as well as her best friend. She is very close with Rias, displaying a master-servant relationship in public while maintaining their friendship in private, but now she is one of Rias' strongest rivals for Issei's affection since she fell in love with him after he told Akeno that he still liked her despite being part Fallen Angel. Their fights over him include seducing him and using their demonic powers against one another. Despite this, the two of them still care greatly for one another.
Kiba is Rias' first Knight. Due to his past where he was part of a project called "The Holy Sword Project", he was led to his death. After seeing him die, Rias thought that it would be a waste of talent so she revived him to become her Knight. She was also the one who gave him his name, "Yuuto Kiba".
Koneko is Rias' first Rook. She cares a lot for Koneko since she was one of her earliest servants, but is considered her love rival since Koneko is truly in love with Issei. Koneko fell in love with Issei in Volume 5 because he helped her overcome her powers that she feared to use. In Volume 11, Koneko proposed to Issei, stating that she wants to become his bride.
Asia is Rias' second Bishop. Rias treats her like a younger sister since they live together but they are also rivals for Issei's love since Issei also cares about her, but she cares for her a lot. She is the only person Rias lets sleep with Issei (besides herself) and end up doing various things with and to Issei due to Asia not wanting to be left out. Both Rias and Asia also show the same interest as they both found kid Issei very cute and work together to turn him into a child for a day.
Xenovia is Rias' second Knight. She is a trustworthy servant but she considers her as a love rival since she wants to have Issei's children and because she develops feelings for Issei as the series progresses.
She is not a member of her peerage but is part of the Angel's representative of the Allied Forces and is a member of Michael's peerage. She considers her a good friend but also sees her as a rival since she is Issei's childhood friend and has a crush on Issei, and now has an intimate love for him. After approaching Irina about her childhood with Issei, the two become closer, as Rias no longer addresses Irina with an honorific.
Rias' first Bishop. Because his powers were unstable prior to the series, he was sealed in the Academy by Rias, and was not seen again until Volume 4 of the light novels. Rias cares for Gasper like a little brother, getting angry at the Khaos Brigade for kidnapping and using him as a tool during the peace treaty between the Three Factions.
Rossweisse is Rias' second Rook who is very loyal to her. Their relationship started when Rias showed her a contract detailing how much a reincarnated Devil would earn perks she would receive. This struck a certain cord inside Rossweisse that appealed to her financial eccentricity. She is also a love rival, since she fell in love with Issei in Volume 17.
Azazel and Rias' relationship isn't clear, but they seem to have a friendly relationship with one another. However, Rias gets annoyed at Azazel whenever he uses Issei as an experimental subject or gives him ideas to lose his virginity, but does seem to still have a good relationship with him.
Rias and Sona are rivals, and they often compete in matches. Sona is one of Rias' best friends. She is jealous of Issei because he was able to help Rias overcome many of her problems that she wanted to help Rias out with, but because of her social standing as a High-Class Devil, she couldn't help her out, and yet Issei did it so easily for her. She congratulates Issei for his promotion recommendation to Middle-Class Devil and tells him to protect and cherish Rias and she considers him to be a very good friend for all that he has done for her. She also asks Issei to call her by her first name like Rias.
Prior to meeting Issei, Rias was engaged with Riser. However, Rias was against this and started a Rating Game with him. As Issei is about to be killed by Riser, Rias pushed Riser out of the way and agrees to marry him. Soon after, Issei wakes up in his bedroom and makes a deal with the Red Dragon, giving the dragon his arm for 10 seconds of his Scale-Male Armor. He also receives a flyer and a message from Sirzechs Lucifer, saying that he can use the flyer to get into the underworld and stage a rescue attempt for Rias. But, before Issei went into the underworld, he asked Asia for her old cross and some holy water. After the 10 seconds Issei had of the Scale-Male Armor, Issei falls to the ground, being held up by Riser. Riser claimed that the battle was over, and that Rias would have to marry Riser, but Issei stated that the battle wasn't over, and opened the bottle of holy water to transfer his energy into the holy water which then started burning Riser's face and eliminated his ability to self-heal, leaving him vulnerable for an attack. Issei dodges Riser's attack and transfers his aura into his Red Dragon gauntlet, giving his attack more power. Issei strikes Riser with the skills everyone has taught him while training and the cross Asia gave to him. Then, Sirzehcs calls it a game and Issei is declared the winner of the battle, leaving on a Gryphon, kissing his reward, Rias.
She is Riser's younger sister, and also another rival for Issei's love and moved in with him in Volume 10 of the light novels. She does not mind as long as she is with him.
Sirzechs is Rias' older brother. Rias grew up with having both feelings of admiration and inferiority towards her brother Sirzechs. When Rias was very small, she called Sirzechs "Onii-sama", while Sirzechs called her "Ria-tan". Sirzechs also loves his sister very much that he is considered to have a sister complex. His love for her was so great that he secretly took pictures of Rias sleeping when she was a child. Sirzechs would cry in tears when he watched videos of Rias in her childhood. Because Rias became older, he passed this role to Issei, though Rias was upset and said she didn't mind Issei doing it, but heavily rejected Sirzechs with doing such things. Despite being dismayed by her brother's excessive show of affection and pampering, Rias truly loves her brother, as shown when she sadly shed tears upon learning that Sirzechs was amongst the three Satans that sacrificed themselves to seal away Trihexa, showing that she deeply cares about him.
Grayfia Lucifuge is the Maid of the Gremory Clan, the wife and Queen of Sirzechs Lucifer, and Rias' older sister-in-law. While Rias loves and admires Grayfia, she also feels inferior to her. When Grayfia is doing her role as a maid, they have the proper relationship of master and maid and Rias would call Grayfia by name. Though when Grayfia has her day off, she returns to being Sirzechs' wife and Rias' sister-in-law. Rias is very scared of Grayfia at this time, to the point of not being able to talk back to her during conversations. Grayfia is called "Onee-sama" during this time. After the events of Volume 21, Rias has shown to be very worried about Grayfia who was shocked and sad that Sirzechs was sealed away in the Isolation Barrier Field.
Another sign of Rias inferior feelings toward Grayfia was when Grayfia was bathing with Issei while drunk. Instead of becoming upset, Rias got teary eyes and acted like a child, thinking that Grayfia had stolen Issei and wouldn't be able to fight back. But this was the first time when she gangs up with Akeno against Grayfia to "win" back Issei.
Zeoticus is the current head of the Gremory Clan. He is the father of Rias and Sirzechs. He planned the engagement of Rias and Riser. Though after the engagement party was crashed by Issei, he reflected on his mistake of forcing his beliefs onto his daughter. He fully supports the relationship between Rias and Issei, even going as far as considering him part of the family already.
It was revealed that he also has great love for his daughter when she was very young. He and Sirzechs would cry with joy and have multiple nosebleeds by looking at how Rias acted when she was an infant. They also took many video recordings of Rias.
Venelana Gremory is the mother of Rias and Sirzechs. She is very strict with Rias due to her status as the next leader of the Gremory but still cares deeply for her daughter. She had urged Rias to further her relationship with Issei such as suggesting the two get married. She supports the relationship between Rias and Issei, as well as considering him part of the family already, like her husband, Zeoticus Gremory, has.
Millicas is the son of Sirzechs and Grayfia. As such, he is Rias' nephew and the next heir of the Gremory Clan after Rias becomes the head. Even though Rias is his aunt, he refers to her as "Rias-neesama", where nee-sama means "older sister". This may be due to their age difference being that of siblings rather than aunt and nephew.
Sairaorg is Rias' cousin from the Bael Clan. This is because Rias' mother was originally from the Bael Clan. Sairaorg was the person who snapped Rias back to her senses (by berating her) after being left completely devastated and heartbroken from hearing that Issei had "died" in the Dimensional Gap.
Misla Bael is the mother of Sairaorg and the aunt of Rias. Rias is strongly sympathetic towards Misla, due to her being terribly persecuted and scorned by the Bael for birthing Sairaorg without the Power of Destruction. Rias even shed tears when she told Issei about Misla and Sairaorg's painful past.
Rias and Bikou are on bad terms due to Bikou constantly making fun of Rias, calling her "Switch Princess".
Rias had developed a close relationship with Issei's parents after allowing her to stay with them, she even calls them "Mother" and "Father". They are supportive of her relationship with their son while hoping to have grandchildren some day.