Itsuya is the identical twin brother of Setsuya, physically indistinguishable from him. Before the story begins, he was imprisoned in the basement of an isolated mansion. He overpowered Setsuya, exchanged places with him, and escaped confinement. This act simultaneously restarted the mansion's previously suspended flow of time.

After taking Setsuya's position, Itsuya intentionally activated a supernatural event by making a large, chained grandfather clock chime. This triggered immediate and overwhelming sexual arousal in every woman residing in the mansion. Itsuya treats these women with consistent cruelty and mistreatment, a stark contrast to Setsuya's kindness. His abusive behavior gradually reveals his true identity to the mansion's inhabitants.

Itsuya's actions initiate a specific consequence linked to the mansion's clock: if it strikes thirteen times, one person within the mansion permanently disappears. His motivations stem from malice and a desire to inflict suffering, targeting both his brother—left bound and helpless in the basement—and the mansion's residents. Throughout the narrative, he shows no redemption or meaningful change.