Sōsuke Yamazaki, born September 14, grew up near Rin Matsuoka. They shared classrooms at Sano Elementary School and pools at the Sano Swim Club, forging a deep bond through swimming and holidays spent with Rin's family. Despite this closeness, a fundamental rift emerged in elementary school when Yamazaki declared his preference for individual events over relays, disliking shared responsibility for wins or losses. Though their friendship endured, communication ceased after Rin transferred to Iwatobi Elementary. Yamazaki continued competitive swimming at Sano Junior High, relentlessly training to match Rin's level. His dedication earned him national recognition in butterfly, but persistent overtraining resulted in a debilitating shoulder injury during his first year at Tokyo's Kujou High School. He concealed the injury while intensifying workouts, reaching a point where no local competitor could defeat him, even as his shoulder worsened. Learning of Rin's return from Australia, Yamazaki undertook daily physical therapy to rehabilitate his shoulder before their reunion. Despite his efforts, his condition deteriorated, forcing him to quit swimming by the summer of his second year. Witnessing Rin's passion during a medley relay with Iwatobi High School reignited his own desire to swim. He transferred to Samezuka Academy for his third year, becoming Rin's roommate and rejoining the swim club. Initially urging Rin to prioritize individual Olympic goals over friendships, Yamazaki later requested to join Rin's relay team, recognizing Rin's need for supportive teammates. Rin adjusted team criteria to prioritize desire over times, granting Yamazaki a place. Competing in the national tournament further damaged his shoulder, culminating in his withdrawal from the final relay. Facing the end of his swimming career, he planned to work at his family's restaurant, "Ichi Yama," after graduation. A farewell match and Rin's encouragement prompted him to reconsider. Post-graduation, Yamazaki underwent successful shoulder surgery in Tokyo and rehabilitation. He resumed competitive swimming, switching to backstroke to reduce shoulder strain. His dedication led to selection for the Japanese National Swim Team. He competed in the Fukuoka Tournament as part of the 4x100m medley relay alongside Rin Matsuoka, Haruka Nanase, and Ikuya Kirishima, achieving first place. Physically, Yamazaki stands tall (185-186 cm) with a muscular build (76 kg), teal eyes, and short dark brown hair. Typically reserved and stoic, he can appear intimidating but possesses underlying kindness and protectiveness, especially towards Rin and Gou Matsuoka, whom he views as a sister. He maintains a dry, harsh perspective on swimming discipline for himself and others, yet actively mentors teammates like Aiichiro Nitori. Outside the pool, he displays a laid-back demeanor, struggles with directions, enjoys cola, pork cutlet, training, and sleeping, and dislikes strong-smelling foods. Academically, he excels in math but struggles with English and history.