Mio Shimazu, daughter of Sakaimachi City's mayor, attends Sakaimachi Academy as a third-year student while leading the historical research club. Her prominent family background invites occasional "rich girl" labels, which she actively resists. She harbors a deep passion for ancient history, focusing on historical cycles and local mysteries, driving her investigation into Ken-no-Misaki and Takano Amahara after studying the book "LEGACY" by historians Kodai Arima, Kouzou Ryuuzouji, and Yuriko.

Physically, she has orange eyes and waist-length orange hair, with updated designs adding side curls and a ribbon.

Romantically drawn to classmate Takuya Arima, Mio openly confesses her feelings but later distances herself upon witnessing his closeness with Mitsuki Ichijou, a teacher and his former lover. She mocks Takuya as "the walking revision," establishing belligerent sexual tension between them. While Masakatsu Yuuki pursues her, she maintains a platonic friendship.

Personal turmoil arises when rumors spread about her father's alleged corruption scandal with Geo Technics, causing her such distress that she considers quitting school or studying abroad.

Her narrative diverges across adaptations: trapped with Takuya under Triangle Mountain in one storyline, she initiates intimacy believing escape impossible. In an adult anime version, she grapples with jealousy after Takuya marries Sayless. Later, Mio conceives Kanna Hatano. Tragedy follows when the present-day Kanna dies in a shrine ritual, displacing Mio to a distant past timeline where she gives birth to Kanna.

Certain routes conclude her arc with a bittersweet return to the story’s starting point alongside others, leaving circumstances unresolved.