Natsuya Kirishima is the older brother of Ikuya Kirishima. He served as captain of the Iwatobi Junior High School Swim Club during the first year of members Haruka Nanase, Makoto Tachibana, Asahi Shiina, and his brother Ikuya. He recruited them, playing a pivotal role in their early swimming development. His age progresses from 14-15 during High☆Speed! events to 19-20 in Free! The Movie -Take Your Marks- and 20-21 in Free!-Dive to the Future-. Physically, Natsuya is relatively tall and muscular, possessing vermillion eyes and short, unruly brown hair. His height increases from 168 cm in junior high to 185 cm during college. His motif animal is a tiger shark. Personality-wise, Natsuya is boisterous, keen, and upbeat, though his best friend Nao Serizawa describes him as socially awkward. He shows vulnerability around Nao but maintains a determined exterior. As swim club captain, he is kind yet firm. He exhibits high competitiveness, notably challenging Haruka Nanase to a race after Haruka declares he only swims freestyle; the race ends in a tie, earning Natsuya's respect for Haruka's ability. Natsuya deeply cares for Ikuya. During their childhood, he encouraged Ikuya's swimming but later distanced himself while Ikuya was in elementary school, intending to foster Ikuya's independence. This unexplained distancing caused Ikuya significant distress and fractured their relationship for several years. After junior high, Natsuya left Japan to attend high school in the U.S. for two years, contributing to their estrangement. Their reconciliation occurs when Ikuya enters university, where Ikuya confronts his weaknesses and repairs their bond. Following high school graduation, Natsuya becomes a freelance competitive swimmer, earning a living through individual races and competing internationally. He maintains his friendship with Nao Serizawa, who transitions into coaching and sports medicine. Natsuya has a low alcohol tolerance despite enjoying drinking. Trivia includes his August 20th birthday and Leo zodiac sign traits (passionate, generous, stubborn). He dislikes math and has an aversion to shiitake mushrooms, though he eats them around Ikuya to set a good example. His favorite quote is "Victory only comes to those who believe in themselves" by Mikhail Makarovich Nitori. Character design notes describe his musculature as "type 4," noticeably muscular with "hard" muscles.