Mario is a classmate in Class 1-2 at the high school where the story takes place. He is a member of a delinquent group that initially includes Atsushi and Kenken. Mario is recognizable by his old Japanese topknot hairstyle, and he also wears earrings and has a chubby build. Among the three delinquents, he is described as the calmest and most intelligent.

His initial role in the story is that of a minor antagonist. Motivated by envy toward Sakamoto’s popularity, Mario and his friends attempt to bully Sakamoto on multiple occasions, but every scheme fails. A turning point occurs when the trio accidentally starts a fire in the school laboratory. Sakamoto takes the blame for them, using his quick thinking to summon a teacher to extinguish the flames. This act of generosity causes the three delinquents to begin respecting Sakamoto, and Mario’s stance shifts from rivalry to admiration.

Mario’s key relationships are with his fellow delinquents Atsushi and Kenken, with whom he shares a close bond, and with Sakamoto, toward whom he develops a newfound appreciation. His character development is minimal but clear: he moves from being an envious troublemaker to a neutral or friendly classmate. His notable abilities include a relatively calm demeanor and greater intelligence compared to his group, though he does not display any extraordinary skills beyond those traits. Overall, Mario serves as part of the comedic antagonistic force that highlights Sakamoto’s effortless coolness.