Kōji Inagaki is a student at Jougaoka High School who excels at sports and is frequently approached by various athletic clubs, though he turns down all invitations. Instead, he dedicates his time to pursuing Atsushi Arisawa, his childhood friend and the object of his persistent romantic affection. Originally from Osaka, he lives with his mother and has an older sister named Akiko, who has moved out and has two children of her own. In his early childhood, he was close to Atsushi, often protecting him, and the two exchanged a first kiss and a promise to marry. After moving away at a young age, Inagaki is thrilled to discover that Atsushi now attends the same high school, and he immediately resumes his pursuit. He possesses a loud, blunt, and forceful personality, often physically imposing himself on Atsushi by attempting to kiss him, which contributes to the chaotic and comedic atmosphere of Atsushi's school life. His role in the story is that of a persistent rival for Atsushi's attention, serving as a source of both humor and tension as Atsushi navigates his confusing feelings for the school nurse, Masayoshi Shibata. Inagaki is highly motivated by his childhood promise and his unwavering infatuation, and his athletic abilities make him a physically energetic and determined presence. His key relationship is with Atsushi, whom he chases relentlessly, and his development largely revolves around learning to navigate his intense feelings within the evolving circumstances of their friendship.