Rei Kawarazaki is a member of the aristocratic Kawarazaki family, occupying a European-style mansion in Japan. This family maintains a disturbing system where deceased members are instantly replaced by others who adopt their identities and roles, creating an atmosphere of unsettling continuity.

Rei entered the family under distressing circumstances, implied to be sufficiently traumatic to naturally provoke desires for escape. Her status remains ambiguous, with suggestions she might seek freedom from the family's cycles of abuse and perversions. However, her trustworthiness is questionable, as she may be complicit or inherently "evil" due to her upbringing and actions within the family structure.

She actively corrupts and manipulates outsiders drawn into the family's orbit, notably Saitou Rokurou, a college student working as their servant. Rei attributes her actions to her corrupted nature, stating she acts as "a brat" who "wanted to corrupt an innocent man." Her methods include psychological manipulation and sexual coercion, aligning with family practices.

Rei endures significant physical and psychological trauma, undergoing invasive experiences like forced drug injections that induce a near-doll-like state, and bodily violations leaving visible "tan-traces." These experiences underscore her victimization within the family system despite her ambiguous role.

In the original narrative's conclusion, Rei enables Rokurou's escape by confronting the family with a shotgun as a distraction. She does not flee with him despite accessible exit routes, leaving her ultimate fate and reasons for staying unresolved. Rei reappears in the sequel series *Kawarazaki-ke no Ichizoku 2 The Animation*, which explores time-twisting elements and the family's continued horrors, though no specific development or additional backstory for her is detailed beyond her presence.