Natsuya Kirishima serves as the captain of the Iwatobi Junior High School Swim Club, recruiting first-year students Haruka Nanase, Makoto Tachibana, Asahi Shiina, and his younger brother Ikuya Kirishima to form the team's core. This leadership role is depicted in the light novel "High☆Speed!" and the anime film "High☆Speed! －Free! Starting Days－". Born on August 20, Natsuya displays a boisterous, keen, and upbeat personality. His best friend Nao Serizawa observes underlying social awkwardness beneath his confident exterior, revealing vulnerability in their private interactions. As captain, he balances kindness with firmness. He notably challenges Haruka Nanase's declaration to swim only freestyle; their resulting tie earns Natsuya's respect for Haruka's abilities and motivates him to continue monitoring and challenging the younger swimmer. Natsuya's relationship with his younger brother Ikuya is defining. Initially close, he deliberately distances himself during Ikuya's elementary school years, believing this will reduce Ikuya's dependency. He implements this decision without explanation, causing Ikuya significant distress and fracturing their relationship for years. After high school, Natsuya becomes a freelance competitive swimmer, participating in events such as the 400-meter individual medley. As an adult, he stands 185 cm (6'1") tall with vermilion eyes and short, unruly brown hair. His physique is noticeably muscular and "hard", classified as "type 4" in production materials, with a tiger shark motif reflecting his competitive nature. He reappears during college years at Hidaka University, where interactions with Ikuya and Haruka facilitate reconciliation. This involves addressing past misunderstandings and rebuilding trust with Ikuya. Personal traits include low alcohol tolerance despite enjoying drinking, a dislike of mathematics and shiitake mushrooms (though he eats the latter to set a good example for Ikuya), and a favorite quote: "Victory only comes to those who believe in themselves."