Isaku Ito is the janitor at Otori Gakuen, a role shadowed by persistent negative rumors among students and staff. He appears as an adult male with short black hair, visible stubble, and a pale complexion, often perceived as unattractive. His attire consists of practical, unremarkable clothing like jackets, tank tops, sweatpants, and occasionally a towel, suitable for maintenance work.

His personality combines a violent disposition and predatory behavior with above-average intelligence, which he uses to orchestrate schemes. His actions reveal a perverse nature, including confinement, kidnapping, and sexual violence. He operates as the primary antagonist, exploiting his access to school facilities.

At summer vacation's start, Isaku traps students—including Kotomi Asakawa, Miyuki Sakaki, and Rika Akagawa—inside the condemned old school building. He boards the exits to prevent escape and systematically targets female students for violation and psychological torment. He employs surveillance, circulating captured videos to intensify fear among the captives.

This portrayal features Isaku as a near-silent physical threat who communicates minimally, relying entirely on physical dominance. His reign concludes through exhaustion after prolonged violent activity, resulting in permanent incapacitation that ends the threat. The aftermath leaves survivors with significant trauma.