Kayoko Kasuga functions as the primary antagonist within her originating narrative. As the sister of the deceased Satoshi, she acts as guardian to a boy named Manabu. Kayoko first appears at Satoshi's funeral, immediately blaming his widow, Honami, for his death and attempting a physical assault, halted only by Manabu's intervention.

Consumed by rage and a drive for vengeance, Kayoko manipulates Manabu into raping Honami, intending to force Honami from the household. This strategy collapses when Honami refuses to leave. Kayoko later accepts an invitation to Honami's home, where Honami explains her decision not to report them and pledges to honor Satoshi's dream of a happy family. During this meeting, Honami permits Kayoko and Manabu sexual access to her. Kayoko subsequently participates in raping Honami, aided by Manabu.

Kayoko's campaign intensifies during a visit to Satoshi's grave. She brings Honami, dressed in a revealing morning gown, and employs a vibrator on her at the gravesite. Kayoko compels Honami to falsely confess responsibility for Satoshi's death before his grave. Following this coerced admission, Kayoko engages in sexual activity with Honami.

A pivotal shift occurs when Manabu attempts to directly rape Kayoko. She verbally defies him during this assault. Manabu redirects his aggression back onto Honami while Kayoko observes approvingly. Manabu eventually ceases raping Honami, declaring his love for Kayoko. This declaration culminates in both Manabu and Honami raping Kayoko, transforming her role from principal aggressor to victim.