Yuto Kosaku is a second-year wing spiker at Inarizaki High School, wearing jersey number 13. He stands 181 cm tall with short brown hair, thick eyebrows, and grey eyes. While taller than the average Japanese high school student, he is shorter than most of his teammates. His name derives from the Corsac Fox, known as "kosakku gitsune" in Japanese, though his name uses a single "k". Kosaku is friendly and competitive, actively encouraging teammates during matches, such as cheering for Heisuke Riseki. His key skill is a powerful jump serve, notable for its speed and sharply angled trajectory, making it difficult to receive. This serve earned him the role of pinch server during critical moments. In the Tokyo Nationals Arc, Kosaku was substituted into the match against Karasuno High near the end of the first set, replacing Omimi. During this rally, he attempted to target Karasuno's Ryunosuke Tanaka but failed and apologized to his teammates, who did not blame him. Subbed in again during the second set, he successfully executed his plan to aim at Tanaka, visibly delighting in the accomplishment. Following Inarizaki's loss to Karasuno, he bowed with his team in acknowledgment of the outcome.