Kōichi Jōnouchi is known as "God's Hall Monitor" at his school, where conflicts are settled through fights. He faces school bully Saburō Ibuki in a boxing match to decide who will date Yukari Takamura. Jōnouchi wins using his signature "Insect-Squashing Punch," despite Ibuki using underhanded tactics.

Jōnouchi relies heavily on this powerful punch. After his victory, Yukari dislikes the move's name and convinces him to rename it the "Jōnouchi Koichi Killer Golden Victory Finisher." This lengthy name proves a strategic flaw during his rematch against Noboru Takizawa; the time taken to announce it leaves him open to a counterattack, contributing to his defeat.

Years later, Jōnouchi becomes principal of Hanebi Highschool, appearing as a six-foot-tall plastic action figure—a form rarely acknowledged. He trains seven transfer students, all named Kakeru, to combat corruption within the education system. During a mission, a package bomb kills him. Even as he is destroyed, he calmly instructs the Kakerus to avenge him. Near the series' conclusion, he undergoes a transformation from plastic into an actual human being.