Big Toenail of Satan presides over Sparta Academy, an institution notorious for its severe disciplinary practices and administrative corruption. He perpetually wears a distinctive white demon mask and a jester hood, obscuring his face for the majority of the series. His true visage is unveiled in the manga’s conclusion, mirroring the mask itself and marked by a birthmark shaped like a foot with pointed toenails.

He displays deeply perverse and sadistic behavior, deriving sexual gratification from the humiliation and punishment of female students. Mayumi Takahashi is a recurring victim of these acts, often used as bait to draw the superheroine Kekko Kamen into confrontations. His methods involve deploying punishment counselors and specialized teachers to execute torturous schemes, though these are invariably disrupted by Kekko Kamen’s interventions.

Despite his absurd appearance and conduct, he exhibits shrewd intelligence. He accurately infers that multiple individuals share the Kekko Kamen identity, suspecting they are concealed among the students and faculty. His strategies occasionally involve manipulating students to turn against Kekko Kamen, and he sometimes displays unexpected competence within the academy’s chaotic environment.

The character appears in media beyond the main series. In the spin-off "Maboroshi Panty," he becomes principal of another school plagued by high crime rates, bringing his team of sadistic teachers and punitive methods with him. He also makes cameo appearances in other works, such as the live-action film adaptation "Cutey Honey 90s" and the "Hanappe Bazooka" OVA, where he is featured among groups of characters.

His role remains consistent across adaptations, invariably cast as the main antagonist who orchestrates events but is ultimately defeated. The manga concludes with his downfall prompting the shutdown of Sparta Academy after a formal investigation.