Isaku Itou works as Oran Academy's janitor, a position shadowed by enduring negative rumors. He is the Itou Family's eldest brother and functions as the primary antagonist, imprisoning high school students—including Kenta, Miyuki, Kotomi, Rika, and their teacher Takashima Kumi—within an abandoned school building. He boards all exits to trap the group, then methodically targets and violates female students, beginning with Rika.

He presents as a senior-aged man with black eyes, short black hair reaching his neck, and frequent facial stubble. His complexion is pale, and he is typically deemed unattractive. He wears practical attire like tank tops, sweatpants, jackets, and sometimes a towel, often wielding a knife.

Personally, he is a perverse figure who commits violent, sadistic acts such as kidnapping and confinement. Despite his brutality, he displays cunning intelligence in strategically setting traps and manipulating the abandoned building's layout to isolate victims. His janitorial role grants him intimate knowledge of the school, which he leverages to control and terrorize the captives.

His background notes membership in the Itou Family, though familial specifics remain unexplored. Across media—including the original visual novel and OVAs like "Isaku," "Isaku: Respect," and "Isaku: Tsumi to Batsu"—he consistently orchestrates the students' captivity and their desperate bid to evade him.