Mayumi Katahira is a young woman who appears in the series Kamen Rider Agito as a university student and the former girlfriend of Ryo Ashihara. She was a member of Professor Yoshihiko Misugi's seminar and through that connection briefly served as a tutor for Mana Kazaya. Her background is marked by personal tragedy: her father, Hisao Katahira, was killed by the Snake Lord Anguis Masculus, one of the monstrous Unknown. Shortly after her father's death, Ryo, who had quit both the swimming club and university, came to see her. The two began to rekindle their relationship, but their reunion was cut short when the Snake Lord attacked them. During the attack, Mayumi witnessed Ryo transform into Kamen Rider Gills, a grotesque and monstrous-looking form. This sight filled her with terror and confusion. She came to believe that she and her father had been targeted because of Ryo's transformation, and that his very existence had brought danger into her life. Unable to overcome her fear and distrust, she rejected Ryo and returned to her hometown of Kagoshima, effectively ending their relationship.

Mayumi's personality is defined by her initial warmth and lingering feelings for Ryo, but also by her vulnerability and deep-seated fear when confronted with the supernatural. She is not a fighter; she is an ordinary civilian caught in extraordinary circumstances. Her motivations are driven by a desire for safety and normalcy, which ultimately leads her to distance herself from Ryo and the world of Agito. Her role in the story is primarily as a catalyst for Ryo's character development, highlighting the personal cost of his curse and the loneliness he endures. Although she is identified as an Agito carrier—meaning she possesses the latent seed of the Agito power within her—she never awakens or demonstrates any special abilities in the series. Her departure marks a turning point for Ryo, reinforcing his isolation and his struggle to accept his own transformation.