Ryūjirō Kotobuki, a third-year mechanical engineering student at Izu University, stands as a prominent co-leader of the Peek a Boo diving club alongside Shinji Tokita. He possesses a tall, muscular frame at 188 centimeters. Initially slimmer, his physique develops over time to match Tokita's robust build. His bright blond hair is frequently slicked back during his part-time work as a bartender.

Kotobuki balances energetic partying with dedicated diving leadership. He often initiates raucous club behavior involving heavy drinking and nudity, yet demonstrates seriousness and extensive knowledge when instructing junior members on diving safety and ocean fundamentals. This duality extends to his skilled bartending; he listens attentively to customers, crafts personalized cocktails, and gains significant popularity, particularly among female clientele.

His relationships center on deep club camaraderie. He shares a close, heterosocial partnership with Tokita, jointly mentoring juniors like Iori Kitahara and Kōhei Imamura—pressuring them into drinking games while offering steadfast guidance during diving training. Though his party persona aligns with Tokita, Kotobuki exhibits distinct charisma in diffusing tense situations or leading group initiatives. He maintains respectful boundaries with female members, intervening against external harassment despite the club's internal nudity norms.

Athletically proficient beyond diving, Kotobuki displays notable tennis abilities, executing advanced techniques like two-handed jumping smashes. His character growth encompasses physical development and evolving mentorship, shifting from instigating chaotic gatherings to fostering team cohesion on diving trips. An incident involving a student crisis highlights his role in university cover-ups; his intervention helped manage a volatile situation, later leveraged by administrators to portray the club favorably.