Aoi is a passive and loyal woman who does not interfere with her husband's affairs, described by Kirei Kotomine as the ideal version of a traditional wife.
She gives off the feeling of a well-raised young lady who became a mother, and she was designed to be a different type of "ideal mother" than Irisviel. Irisviel embodies a "fun mother", while Aoi is a "peaceful mother" who is better at using the "hidden power" that is inherent to mothers. She gives off the strong feeling of "That husband of mine, never taking care of himself", and while she looks like Rin at a glance, her behavior is more like that of Sakura.
Formerly known as Aoi Zenjou (禅城 葵, 'Zenjō Aoi'?), she is the childhood friend of Kariya Matou. She is a descendant of the Zenjou family, which had been a family that specialized in maximizing the magical pedigree of their partners, though the Zenjou have since become a family of ordinary people and Aoi herself has no magical circuits. She became the wife of Tokiomi, adopting his surname, and gave birth to two daughters, Rin and Sakura, who both possessed great inherent potential as mages.
She accepted Tokiomi's proposal with full knowledge of what it meant to marry a mage. While Kariya loved her as well, he peacefully gave up pursuing her affections because she seemed happy accepting Tokiomi's proposal. It is implied that Aoi and Kariya remained in touch, given that her daughters, especially Sakura, were fond of Kariya's company. Tokiomi's sudden decision to give Sakura away to Zouken Matou comes as a shock to Aoi, though she does not openly oppose his decision. She leaves at the beginning of the Holy Grail War with Rin to live at her parents' home for the duration of the conflict.
During the Holy Grail War, Aoi is forced to return to Fuyuki City to look for Rin, who had run away to find her friend Kotone, who earlier had been kidnapped by Caster. Later on, Kariya gives Rin back to Aoi after he finds her, which is also how Aoi finds out about his condition and his resolve to kill Tokiomi.
On the final day of the war, Aoi finds her husband dead, with Kariya present in the room, however, being unaware that Kirei was responsible for Tokiomi's death and that Kariya had arrived to confront Tokiomi only after the man was dead. In her grief, she blames Kariya and the Matou family for taking away both Sakura and her husband and claims that Kariya never loved anyone. With both his resolve and heart crushed, he violently strangles Aoi.She survives the war, but is confined to a wheelchair and suffers from brain damage, not understanding that Tokiomi is dead and Sakura is gone. Rin takes care of her mother, but feels lonely as her mother's condition leaves her isolated. Aoi appears to have died before the events of Fate/stay night, leaving Kotomine as Rin's sole guardian.
In accordance to Tokiomi's will, Aoi inherits the properties and the estates of the Tohsaka family in trust for Rin. Because of Aoi's mental deterioration, Tokiomi's estate is actually managed by Kotomine even after Aoi's presumed death, when the estate is inherited by Rin.