Natsuya Kirishima is a supporting character who first appears in the second volume of the light novel High Speed and later in the prequel film High Speed Free Starting Days. He is the older brother of Ikuya Kirishima. During his junior high years, Natsuya served as the captain of the Iwatobi Junior High School Swim Club, where he played a pivotal role in bringing together and developing the skills of Haruka Nanase, Makoto Tachibana, Asahi Shiina, and his brother Ikuya. Known for his friendly, lively personality and strong leadership, he was widely admired by his teammates. Standing at 185 centimeters, Natsuya is an exceptionally talented swimmer who eventually becomes a freelance competitive swimmer after high school. He is passionate and energetic, often challenging others and approaching life with a bold, reliable demeanor. His relationship with his younger brother Ikuya is complex; he cares for him deeply but once distanced himself in an effort to help Ikuya grow independently, a decision that led to misunderstandings and strain between them. Natsuya shares a close bond with Nao Serizawa, the former manager of the middle school swim team and his longtime best friend, who often serves as a grounding influence. In later installments such as Free Dive to the Future, Natsuya returns to Japan to participate in a joint training camp and acts as a mentor, recognizing Haruka Nanase's natural talent and encouraging him to aim for the world stage. He is known to enjoy fried chicken and curry, dislikes mushrooms, and has a low tolerance for alcohol despite a fondness for drinking. His role in the story is that of a catalyst and mentor, shaping the path of the next generation of swimmers while navigating his own complex family ties.