Yūto Kosaku is a second-year student at Inarizaki High School and a wing spiker/pinch server for the volleyball team, wearing jersey number 13. He stands 181 cm tall, making him taller than average but shorter than most teammates, with short brown hair, thick eyebrows, and grey eyes. His personality blends competitiveness with approachability; he actively encourages teammates like Heisuke Riseki during serves while maintaining focused determination in matches. During the Spring Interhigh Nationals match against Karasuno, Kosaku was substituted for starting middle blocker Ren Ōmimi near the end of the first set. After a rally, he apologized for failing to target opposing player Ryūnosuke Tanaka as intended, though his teammates offered no blame. When subbed in again during the second set, he successfully executed his strategy of aiming at Tanaka, showing visible satisfaction upon achieving this goal. His most recognized skill is a powerful jump serve noted for sharp angles and speed, deployed in critical moments like his pinch server appearance against Karasuno. His name derives from the Corsac Fox ("kosakku gitsune" in Japanese), with his surname using the single "k" spelling variation ("Kosaku"). Available official media, including spin-offs or additional seasons, documents no significant character development or expanded background beyond his participation in the Nationals arc. His relationships with teammates remain functional and cooperative, consistent with Inarizaki’s team-oriented philosophy.