Mizuha is a descendant of Hayase, the ancient warrior who once pursued Fushi, and she is the seventh successor to the Guardians' cult that worships Fushi as a deity. She is a student at Minamoto Junior High School, where she is admired by peers and adults for her academic excellence, athletic ability, and polished demeanor. She comes from a well-off family and lives with her mother, Izumi, a demanding writer who constantly pushes Mizuha to excel and rarely acknowledges her successes. Under pressure from this high expectation, Mizuha internalizes the belief that she must be perfect and comes to view others who struggle as inferior, though she outwardly appears gentle and reserved.

Mizuha’s motivations are shaped by her family legacy and her own desire for freedom. When she discovers a biography of Hayase and sees a picture of Fushi, she becomes fascinated and obsessed, viewing him as a figure who can fulfill Hayase’s long‑standing wish. She runs away from home and seeks out Fushi, asking him to be her friend with an eerie smile. Her encounter with Fushi draws her into a larger conflict involving the Nokkers, parasitic entities that have survived into the modern era. One such Nokker, which once inhabited her ancestor Kahaku, now lurks within Mizuha. This Nokker influences her actions, driving her to a violent outburst in which she attacks her mother, though the woman inexplicably revives.

Throughout the story, Mizuha experiences a fractured sense of self. Her upbringing leads her to maintain a perfect image, but she grows increasingly disillusioned and desperate. After the attack on her mother, she flees in a dissociative state and is found by Fushi, who takes her to his home despite the distrust of his immortal companions. Her presence becomes a central point of tension in the modern arc, representing the unresolved legacy of Hayase and the enduring threat of the Nokkers. She eventually meets a fatal fall, and in the afterlife her individual dream is subsumed by the collective will of her ancestors, particularly Hayase, who congratulates her on fulfilling the family line’s aim.

Mizuha’s key relationships include her best friend Hanna, who supports her; her demanding mother Izumi; and Fushi, who embodies the being her ancestor both worshipped and resented. Her role in the story is to force Fushi to confront whether he can coexist with or must eradicate the Nokkers, as she is both a descendant of his enemy and a victim of the parasitic influence. She undergoes significant development from a seemingly perfect girl to a traumatized young woman whose identity is stripped away in death. Mizuha possesses no supernatural abilities beyond being a human host for a Nokker, but her psychological complexity and the moral questions she raises are central to the narrative’s conclusion.