Kouji Inagaki is a student at Jougaoka High School, originally from Osaka, who lives with his mother. His older sister Akiko has moved out and has children. Inagaki possesses a loud and blunt personality and excels at sports, attracting frequent club invitations which he declines to focus on pursuing his childhood friend, Arisawa Atsushi.

As childhood friends, Inagaki and Atsushi shared their first kiss and a promise to marry, fueling Inagaki's persistent romantic advances and protective behavior toward Atsushi during high school. His aggressive pursuit includes frequent, rejected attempts to kiss Atsushi, creating tension.

Inagaki develops a close friendship with classmate Yoriko Hayase, who actively supports his romantic goals while concealing her own feelings for him. Their dynamic involves belligerent sexual tension, especially as Yoriko supports Atsushi's relationship with Masayoshi Shibata. A pivotal school trip sees Yoriko face an attempted date rape; Inagaki intervenes to rescue her. This event leads to emotional vulnerability between them, catalyzing a shift from friendship to mutual romance.

After graduation, Inagaki matures beyond his obsession with Atsushi, enters the workforce, and maintains their friendship. He eventually marries Yoriko, and they have twins, solidifying his transition into adulthood and family life. Throughout this development, Inagaki evolves from a juvenile delinquent archetype with comedic traits—including distinctive fangs—into a more grounded character capable of emotional depth and commitment.