Kōsaku Sakamoto, a high school student from Kumamoto, relocates to Tokyo after his mother's death. He moves in with her friend, yakuza boss Ibari Ōzora, and Ibari's four children. His father previously drowned in a rice paddy while intoxicated. Initially apprehensive about the yakuza household, Kōsaku reconsiders upon meeting the children, particularly developing an immediate attraction to Hibari Ōzora. This changes when he learns Hibari was assigned male at birth, creating persistent internal conflict as he struggles to reconcile his feelings.

He attends Wakaba Academy alongside Hibari and her sister Tsubame. Academically average and initially physically weak, he lacks fighting skills, making him vulnerable. Seeking distance from Hibari and physical strength, he joins the school's boxing club. Through rigorous training and sparring, especially with rival Makoto Shiina, his boxing skills advance significantly. His club involvement exposes him to challenges like training camps and inter-school matches.

His relationship with Hibari remains central. Despite consistently refusing her romantic advances and publicly denying involvement, he grows increasingly concerned about his deepening emotional attachment. He actively protects her secret, aiding schemes to prevent discovery of her assigned gender, such as Tsubame impersonating Hibari during health checkups. He experiences jealousy when Hibari interacts with suitors like popular student Takuto Honda or mafia heir Michael Corleone. Simultaneously, he develops a crush on boxing club manager Rie Kawai, complicated by Rie's affection for Shiina and Hibari's persistent attentions.

Key events highlight his evolving dynamic with Hibari. During a school festival play, he takes the role of Prince to Hibari's Sleeping Beauty, culminating in an onstage kiss. A supernatural body-swap forces him to temporarily inhabit Hibari's body, intensifying his confusion. He experiences nightmares involving Hibari, reflecting subconscious anxieties. While often vocalizing distress about potentially "becoming a pervert" due to Hibari's influence, his actions show protective loyalty, confronting those who harass her or threaten her secret.

Physically, Kōsaku has black hair cut to ear length and black eyes, typically wearing a standard male gakuran school uniform. His personality is pure-hearted but conflicted, gradually maturing through experiences with the Ōzora family and the boxing club.