A quiet young girl with a rather blunt personality who wears Miko attire.
While she may seem normal, she is also known as “Deep Blood” and has the ability to kill vampires when they approach her, as her blood’s smell attracts them. However, it made her miserable because it killed her family and friends that were turned into vampires. This prompts her to go to Academy City, hoping to find a way to get rid of her powers, until she meets the alchemist Aureolus Izzard. She cooperates with him because he told her that her blood can be used to save the life of a person he cared for (Index) in return for helping her get rid of her powers.
When Aureolus learns Index’s life was already saved by Tōma and tries to kill him, Aisa attempts to defend Tōma, which prompts Aureolus to use his alchemy to try to kill her as she was now useless to him. Her life was saved by Tōma who dispelled the spell on her with his right hand. When she asks Tōma about why he saved her, he told her that he did not have a reason to rescue her. After the incident, she lives with Tōma and Index but later moves to Komoe’s apartment and wears a magical power-blocking cross, which enables her to live what she had always wanted: a normal life. She was formerly a student at Kirigaoka Academy for Girls, one of the five elite schools of Academy City, but was transfer to Tōma’s school when her sponsor, Aureolus Izzard mysteriously disappear thus ending her enrollment at Kirigaoka Academy. Ever since Tōma saved her life, Aisa started to have feelings for him, which is ironic as she once joked that Tōma’s life was like a dating sim.