Ryūjiro Kotobuki is a third-year mechanical engineering student at Izu University and a prominent member of the Peek a Boo diving club based at the Grand Blue diving shop. Standing 188 cm tall with a muscular physique and distinctive bright blond middle-parted hair, he initially appeared relatively slim but gradually developed a robust build mirroring his close friend and fellow senior Shinji Tokita.

His personality blends energetic, party-loving enthusiasm with serious dedication to diving. He actively participates in the club’s alcohol-fueled gatherings, often appearing nude or semi-nude, and pressures juniors like Iori Kitahara and Kouhei Imamura into drinking sessions. Despite this boisterous demeanor, he displays deep knowledge and responsibility in diving safety and instruction, mentoring younger members with patience and attentiveness. As a bartender, he demonstrates professionalism by crafting personalized cocktails, gaining popularity especially among female patrons, and lives by his catchphrase, "Alcohol is humanity’s friend."

Ryūjiro shares a close partnership with Shinji Tokita, co-leading the diving club and frequently appearing alongside him. Their dynamic mixes camaraderie with light rivalry, and they jointly manage critical club responsibilities—such as defusing a campus crisis involving a distressed student through alcohol, an action that inadvertently secured the club’s formal university recognition. He maintains respectful relationships with female members like Azusa Hamaoka and Nanaka Kotegawa, adhering to Peek a Boo’s unwritten rule against non-consensual inappropriate behavior.

His development features increased physical conditioning, evolving from a slimmer frame to a muscular build comparable to Tokita’s. While his core traits remain consistent, his mentorship and leadership solidify through recurring diving activities and club management. His bartending skills and popularity in that role persist as a steady character aspect across all media.