Nao Serizawa co-captained and managed the Iwatobi Junior High swim club, mentoring first-years Haruka Nanase, Makoto Tachibana, Asahi Shiina, and Ikuya Kirishima. A retinal detachment requiring surgery permanently restricted his swimming to under fifteen minutes due to eye pressure, ending his competitive career but fueling his dedication to coaching and sports medicine. Gentle, patient, and soft-spoken, he possessed keen insight into others' motivations. His logical approach pinpointed individual obstacles for each swimmer, crafting tailored improvement strategies like proposing a biaxial swim style for Haruka. Though kind, he upheld strict training standards. His enduring friendship with Natsuya Kirishima included Natsuya's hope they might swim together again if medically cleared. After junior high, he attended Soufukan High School before moving to Tokyo for sports medicine studies at Oudoukan University. He coached children part-time at a local pool while residing in the apartment Haruka Nanase later occupied. Relocation eventually ended his coaching, but reconnecting with Makoto Tachibana—who took over his position—renewed his ties to the former Iwatobi swimmers. Physically slender and toned at 178 cm, he had droopy lime-green eyes and long periwinkle hair dipping between his eyes. Post-surgery, he wore corrective lenses and sported a shorter hairstyle. His hobbies included maintaining a tropical fish aquarium and watching movies or documentaries with Natsuya. He favored extra-spicy noodle soup and mustard lotus but disliked bitter melon. Academically strong across all subjects, he excelled notably in English and mathematics. Before swimming, he practiced Aikido martial arts. Personality assessments noted his dependability offset by occasional forgetfulness, such as misplacing his phone. Born August 27 under Virgo, traits included loyalty, kindness, hard work, criticism, and anxiety. Astrological profiles highlighted strong inner strength and interpersonal skills balanced by a need for self-grounding.