Sosuke Yamazaki, born September 14 under Virgo, stands 185 cm in high school, later reaching 186 cm, with a lean, athletic frame defined by broad shoulders and flexibility. His physique fuels a distinctive butterfly stroke technique, lifting his upper body higher to reduce water resistance. A middle school overtraining injury shifts his focus to backstroke, adapting his career around persistent shoulder strain. Raised alongside Rin Matsuoka in Sano, their bond forms during shared classes at Sano Elementary and Junior High, blending friendship with rivalry. A childhood disagreement over relay accountability steers Sosuke toward individual competitions, fracturing their dynamic until high school. Transferring to Samezuka Academy in his third year, he rooms with Rin, masking his worsening injury while clashing with Rin’s growing emphasis on teamwork. Witnessing Rin’s passion for relays during regionals, Sosuke defies physical limits to join a relay, signaling his gradual embrace of collective effort. Post-graduation plans to manage his family’s "Ichi Yama" restaurant stall after Rin’s encouragement and successful Tokyo surgery revive his career. Rehabilitation pivots him fully to backstroke, easing shoulder strain as he ascends to Japan’s National Swim Team, competing in events like the Fukuoka Tournament and medley relays. Reserved and stoic, Sosuke’s ISTJ personality prioritizes logic and experience, often conflicting with Rin’s emotional choices. Yet he exhibits quiet loyalty, mentoring teammates like Aiichiro Nitori in technique and shielding them from criticism. Beyond the pool, he favors naps, rigorous training, and battles a tendency to get lost. His arc traces a shift from solitary ambition to valuing camaraderie, marked by relay returns and addressing teammates by first names—a gesture of trust. Persevering through injury and recalibrated goals, he emerges as a national athlete and pillar within his swimming circle, balancing competitive rigor with steadfast support.