Seijūrō Mikoshiba, a central figure in the Free! franchise, boasts a decorated swimming career beginning with numerous grade school victories, though he consistently trailed only behind Haruka Nanase. At Samezuka Academy, he rose to swim team captain, specializing in the butterfly stroke while also excelling in long-distance freestyle. His powerful strokes, extraordinary lung capacity, and remarkable stamina enabled him to set competition records, famously completing one hundred consecutive 100-meter races without a loss. As captain, his vigilant leadership prioritized both physical readiness and mental well-being; he removed swimmers from relays if he deemed them unfit. He appointed Rin Matsuoka as his successor upon graduating. Securing an athletic scholarship, Seijūrō enrolled at Hidaka University (alternately called Todoroki University), joining the swim team alongside Haruka Nanase and Asahi Shiina as their senior. A defeat against professional swimmers at a national meet led him to take a year off competitions for intensive self-training, experimenting with breaststroke to rediscover his potential. He maintained ties to Samezuka, frequently returning to support the team during key meets, balancing competition with mentorship. Standing 187 cm (later 188 cm) and weighing 77 kg, Seijūrō possesses a muscular build, red hair swept backward, golden eyes, and a slightly tanned complexion. His image color is vermilion, his motif animal a sea turtle, and his image number 14. He often wears Samezuka Academy's black training jacket with white and red accents, partially unzipped. Outside swimming, he pursues tennis, beach volleyball, and karaoke. His personality blends effervescent charm, especially around girls, with disciplined leadership. He developed an instant infatuation with Gou Matsuoka, calling her "cute" and decorating his room with her photos. Yet he commanded respect as a logical, serious captain, closely monitoring his team. He proved forgiving yet uncompromising, disciplining Rin for swimming with rivals but affirming his captaincy. Valuing fairness and growth, he instituted a team tradition allowing challenges to 100-meter races. Family plays a significant role. His younger brother, Momotarō Mikoshiba, joined Samezuka's swim team and also developed an interest in Gou, sharing a bond of mutual affection and friendly sibling rivalry. His younger sister, Isuzu Mikoshiba, captains her high school swim team at an institution linked to his university. He also shares a playful rivalry with Tsubasa Hoshikawa, captain of Shirokami Academy University's swim team, extending beyond meets to school festivals. Trivia includes his self-proclaimed title "Japan's Legendary Sea Otter" (distinct from his official sea turtle motif), his hobby of stag beetle hunting, and favorite foods: fried eggs, triple-cheese burgers, and chocolate pudding ice cream. Born on December 6 under Sagittarius, his traits reflect the sign's generosity, optimism, and impatience. He owns a notable pink T-shirt referencing the Momotarō folk tale, a nod to his brother.