Saizo's male presentation stems from Kabuki theater traditions forbidding female performers, enforced by her homosexual father who imposed masculine traits during her upbringing. Her father perished in a fire after she accidentally interrupted him with his lover, leading her to interpret her name—linked to the word "sin"—as proof of inherent sinfulness, resulting in lingering guilt.

Trained since childhood in a ritual dance that attracts Yōi (supernatural entities), this art historically connects to luring the goddess Amaterasu from her cave. She wields a paper fan as her combat tool, which unfolds into a binding rope to temporarily restrain Yōi during missions with the demon-hunting group Ayashi.

While investigating disappearances at Kabuki theaters, Saizo rejected assistance and insisted on working alone. This led to her possession by a Mask Yōi, which exploited her dance to draw other Yōi into Edo. The Ayashi leadership consequently ordered her elimination. Rescued during a confrontation that forced her to face her trauma, she ultimately destroyed the Mask Yōi herself. This event became a pivotal moment in her self-acceptance.

Her backstory confirms her age: she states she was not born 15 years before the main events, with the theater fire occurring at age 13 and her joining the Ayashi approximately one year later, placing her around 14 during the series.