The Mystery Principal of Sparta Academy, whose true name is Toenail of Satan, serves as the primary antagonist of the series. He is the head of the corrupt and sadistic boarding school where the story takes place, a position he uses to exercise complete control over the institution and its students.

In terms of background, the Mystery Principal is the architect of the brutal system at Sparta Academy. Under his leadership, the school operates as a place where teachers are encouraged to physically and psychologically torment students, particularly the female ones, deriving sexual satisfaction from their suffering. His appearance reflects his malevolent nature: he dresses in a demonic white mask and jester-like hood, giving him a grotesque and theatrical presence that underscores his role as a villain.

Personality-wise, the Mystery Principal is a despicable figure consumed by lust, cruelty, and a desire for power. He is the embodiment of corruption within the school and represents the type of tyrannical authority that the heroine opposes. He is both perverted and misogynistic, viewing the female students as objects to be exploited rather than as individuals deserving of respect and safety. His actions are driven by a sadistic pleasure in the suffering of others, and he shows no remorse for the systematic abuse he orchestrates.

The Mystery Principal's primary motivation is to maintain his absolute authority over Sparta Academy and to continue the regime of humiliation and punishment unchecked. He is also obsessed with discovering the true identity of Kekko Kamen, the mysterious nude superheroine who regularly intervenes to protect the students from his teachers' advances. His attempts to unmask her and eliminate her interference drive much of the conflict in the story.

In the narrative, the Mystery Principal functions as the central antagonist and the ultimate authority figure that Kekko Kamen must overcome. He does not typically engage in direct combat himself, instead deploying a series of punishment teachers and specialized enforcers to deal with the heroine. He oversees the school's operations from a position of power, issuing orders and scheming to trap or expose Kekko Kamen.

Key relationships within the story include his role as the leader of the corrupt faculty, who carry out his orders in exchange for the freedom to indulge their own perversions. He stands in direct opposition to both Kekko Kamen, who represents justice and protection for the innocent, and the student Mayumi Takahashi, who is the primary target of the school's abuse and the person Kekko Kamen most frequently defends.

As for the Mystery Principal's development over the course of the series, his character does not undergo significant change or redemption. He remains a consistently villainous figure who is repeatedly thwarted by Kekko Kamen, yet continues his attempts to regain control of the school. In the final chapter, the school is shut down thanks to the efforts of Kekko Kamen and her allies, which represents the ultimate defeat of his regime.

Regarding notable abilities, the Mystery Principal possesses the authority and resources of the school's administration, allowing him to command the faculty and deploy various punishment teachers with specialized skills against Kekko Kamen. However, his primary strength lies in his cunning and his willingness to use any means necessary to achieve his goals, rather than in any personal physical prowess. His most recognizable feature is his distinctive demonic jester costume, which emphasizes his role as a theatrical and menacing antagonist.