Birthday: April 9th
Height: 172 cm (5'8")
BWH: 99/58/90
Rank: Top-class Devil
Role: King
Race: Pure-Blooded Devil
Power: Princess of Destruction
First Appearance: Volume 1

A 12th grade student and the prettiest girl in the academy. Comes from the old noble of pure devils, the house of Gremory, a top-class devil who is called a genius nicknamed "Crimson Ruin Princess". Her brother is in fact one of the Yondai-Maou, Lucifer. Both of them possess the power of destruction. She revives Issei as her servant after finding him dead, killed by a fallen-angel and possessing a rare Sacred Gear.

From her mother, Rias has inherited the Bael Clan's Power of Destruction. This power allows her to release energies that can disintegrate matter, both living and inanimate, with relative ease. Opponents who wield sufficiently greater power, however, can shield themselves from this.

Rias has gained the ability to project a red light from her breasts, which recharges Issei's energy. Each use of this power causes her breasts to temporarily shrink in size, which makes Issei cry even though it's sometimes necessary. Her breast size will restore itself after a few days of rest.

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 being 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.