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Rias Gremory (リアス・グレモリー)Age: 17
Height: 172 cm (5'8")
Rank: Top-class Devil
Race: Pure blooded Devil
Power: Power 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.
At the beginning of Volume 2 Rias goes to Issei's room during night time, attempting to have Issei take her virginity in hopes of breaking her engagement. The attempt, however, was stopped with the appearance of Grayfia Lucifuge, Sirzechs' Queen, who scolds Rias for attempting to break off the engagement set by her brother and father. When Rias' fiancé, Riser Phenex, appears in the Occult Research Club, Rias, who was still unsatisfied with the engagement, agreed to have a Rating Game with Riser under the condition that should Rias win, the engagement will become invalid, and should Riser win, Rias would be forced to marry Riser immediately.
In preparation for her Rating Game with Riser, Rias sets up a training camp in the mountains for Issei and Asia who both lack fighting experience. During their training in the mountains, Rias starts to develop strong feelings for Issei due to Issei's feelings for her. On the day of the Rating Game, Rias faces Riser in a replica version of Kuoh Academy set up in a dimensional space. While the battle initially went well for Rias' team, they ultimately lost due to the lack of people on her team, surrendering after seeing Issei being beaten to near-death, yet still standing, trying to fight for her. She was later seen at her engagement party in the Underworld, which was interrupted by Issei who came to challenge Riser one more time with Rias' engagement on the line, with Issei succesfully winning this time. After Issei helps to break off the engagement between herself and Riser, Rias falls in love with Issei and kisses him for the first time. She moves into Issei's house after the incident with Riser.
In Volume 3, Rias introduces her new servants, Issei and Asia, to the Student Council members led by Sona Sitri, a pure-blooded High-Class Devil from the Sitri Clan who is her childhood friend and rival. After Issei noticed Yuuto's change after seeing his childhood photos among one which Issei took with his childhood friend, Irina Shidou and her father who was holding a Holy Sword, she explains Yuuto's past to Issei and was greatly disappointed when Yuuto left her to seek his revenge after seeing both Xenovia and Irina. She and Sona eventually found out the alliance Issei, Koneko and Saji made with the church duo to destroy the Excaliburs, punishing the two of them for their actions.
When Kokabiel, one of the leaders of the Fallen Angels, announced his war declaration against both the Gremory and Sitri clans who are attending the Kuoh Academy, Rias gathers all her servants (except for Yuuto who is still missing) in an attempt to fight back while having the Sitri Group as support. During the fight, Kokabiel summoned out many Cerberus to battle the Gremory Team but was defeated by the combined effort of the Gremory Team and Xenovia who arrived at the battlefield. After Yuuto attained his Balance Breaker and broke the Excalibur with Xenovia's help, the Gremory Team was then forced to battle Kokabiel himself but were easily overwhelmed by the Fallen Angel Leader and was in despair until the sudden interference of Vali Lucifer who came to retrieve Kokabiel under Azazel's order.
In Volume 4, after Issei reports on his meeting with Azazel much to Rias' anger on Azazel's action, they are surprised at the sudden appearance of Rias' brother, Sirzechs Lucifer, who came for Rias' class observation, telling Rias that Azazel won't do any drastic action like Kokabiel. Rias later unseals her other Bishop, Gasper Vladi, a half-Vampire who she revived as a Devil using a Mutation Piece, after receiving orders from the top-brass. During the peace talk between the Three Factions, Rias is asked to report the events that happened during Kokabiel's attack and assisted when the Khaos Brigade attacked using castling to rescue Gasper who was used by the terrorists.
Near the end of Volume 4, Rias is seen troubled when Azazel introduces himself as the advisor for the Occult Research Club, getting angry at Azazel when he suggested to take Issei on a virgin graduation tour, telling Azazel that Issei's virginity is under her watch. She also decide to renovate Issei's house after Akeno and Xenovia moves into the Hyoudou Residence.
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.