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, and Ravel.
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 and although Issei remained oblivious to the situation, he had mentioned that he would like to marry Rias one day.
Akeno is Rias' first servant, her Queen as well as being one of her closest friends, calling each other by their first name when in private. 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 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 over Issei's love and she is considered her biggest rival. 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.
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. 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 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 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.
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.
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. 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, Lord 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 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.
Rias is on good terms with Serafall, as Rias is close friends with her younger sister Sona.
Rias is on good terms with Mephisto as he knows seemingly the entirety of her family, Going at least back to her Great Grandparents.