Issei officially becomes Rias' boyfriend after the latter finally confesses his love for her in Volume 10 of the light novels. He took a long time to do this because he was still traumatized from the incident with Raynare, which was affecting his relationships with all the girls. He was given permission to start a harem by Rias' mother so that all the girls would be happy because they weren't going to give up just because he is in love with Rias.
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 pervertedly gaze on her naked body. Following the events of Volume 2 where she fell in love with Issei, she attempts to keeps him away from the other girls who may have affections for him. This includes Akeno, Asia, Xenovia, Koneko, Irina, and Ravel.
Rias seems to be something one would describe as a gentler version of the classic tsundere. While she does not ordinarily punish Issei using physical penalties, she has inflicted pain on him before. She also became issei's wife lover and sex slave. By calling issei's her one and only true master.
Akeno is Rias' first servant and one of her closest friends, calling each other by their first name when in private. She is very close with Rias, having a master-servant relationship, but now she is one of her strongest rivals for Issei's love since she became her rival in Volume 4 and she fell in love with him in Volume 7 of the Light Novels. Akeno and Rias are bitterly fighting each other for Issei's love and she is considered her biggest rival. Despite so, the two of them cares greatly for one another.
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, Rias thought that it would be a waste of talent so she revived him to become her servant. She was also the one who gave him his name, "Yuuto Kiba".
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. Koneko proposed to Issei that she wants to become his bride in Volume 11 of the Light Novels.
Rias treats her like a sister since they live together but they are also rivals for Issei's love since Issei also loves her, but she cares for her a lot.
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 representation 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 him. Irina actually wants to have a serious relationship with Issei with the possibility of children but she would have a hard time since it could cause her to "fall" and turn her into a Fallen Angel.
Rias' first Bishop. Because his powers were uncontrolled 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 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 position as a High-Class Devil, she didn't have the time to 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 as 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. When Issei was about to be killed, she begged him not to do so and said that she would marry him. She kissed Issei after he defeats Riser during the engagement ceremony.
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 being his concubine as long as she is with him.
Sirzechs is Rias' older brother. Rias growed up with have both feeling of admiration and inferior to her brother, Sirzechs. When Rias was very small, she called Sirzechs "Onii-tama", 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 took secret pictures of Rias sleeping when she was a child. Sirzechs would cry in tears when he watches 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.
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.
Lord Gremory 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.
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.
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.
Misla Bael is the mother of Sairaorg and the aunt of Rias.