Ryūnosuke Oohara, frequently called Ryu, is the sole son of the Fourth leader of the prominent Oohara yakuza family. Enduring social isolation from early childhood due to his family background, he clung possessively to his childhood friend, Naoki Satoya (Nao). This possessiveness drove him to confront anyone perceived as coming between them, including shouting at a girl who liked Naoki that he liked him more and fighting male students over Naoki's attention. Raised under strict guidance to become the family's successor, Ryūnosuke ultimately refused to inherit leadership of the yakuza gang. His reluctance stemmed from fears that his association endangered Naoki's family business and safety, particularly after Naoki suffered a gunshot wound during an attack targeting Ryūnosuke. This event solidified his resolve to prioritize his relationship over yakuza succession.

Ryūnosuke exhibits a duality in personality: outwardly rugged and combat-proficient, capable of defeating multiple attackers alone, yet inwardly displaying a childlike dependence on Naoki. He often seeks physical closeness and emotional reassurance, openly confessing his love and expressing desires for intimacy, though Naoki initially rebuffed these advances. His future plans centered on building a conventional life with Naoki, including pursuing a "katagi" (normal person's) job, and he whimsically considered becoming a train driver, admiring the control of large machinery. Despite his tough exterior, Ryūnosuke reveals vulnerability, admitting he might cry if separated from Naoki and relying on him as a bodyguard during heightened threats.

His development culminates in mutual commitment with Naoki. After Naoki is injured protecting him, Ryūnosuke reiterates his fear that assuming yakuza leadership would force them apart. This prompts Naoki to choose solidarity, leading them to perform a sake cup exchange ceremony—a symbolic bond signifying lifelong partnership, stronger than marriage. Ryūnosuke shifts from seeking Naoki's protection to accepting equality, acknowledging Naoki's strength and vowing to face challenges together without overprotectiveness.