Koutarou Matsudaira is Kazuya Uesugi's best friend and teammate on the Meisei High School baseball team, serving as the team's catcher and clean-up hitter. He forms a key battery partnership with pitcher Kazuya. Physically, he possesses black hair, black eyes, stands 1.78 meters tall, and weighs 92 kilograms with a noticeably robust build.

His background is shaped by his father, Yutaka Matsudaira, a former professional baseball player whose career ended prematurely due to severe injury. This event instills a significant fear in Koutarou of suffering a similar fate. Despite lacking natural athletic talent, he compensates through relentless hard work and dedication, driven by this desire to avoid his father's outcome. Academically intelligent, he maintains a calm, collected demeanor, traits contributing to his leadership qualities and leading to his appointment as team captain during his second year at Meisei.

Initially, Koutarou holds a strong bias against Tatsuya Uesugi, Kazuya's twin brother, viewing him as inferior in character and athletic ability compared to Kazuya. He prioritizes his partnership with Kazuya and openly questions Tatsuya's commitment and skills. However, after Kazuya's untimely death, Koutarou's role evolves. He becomes a crucial part of Tatsuya's support system as Tatsuya steps into the role of Meisei's ace pitcher. While remaining dedicated to honoring Kazuya's legacy, he gradually adjusts to working with Tatsuya, focusing on the team's collective goals.

Koutarou is characterized by his serious, methodical, and organized approach during games. Off the field, he is known for his hearty appetite, occasionally receiving reminders to moderate his eating habits. His development includes learning to balance his grief over Kazuya's loss with his responsibilities as captain, mentoring teammates and strategically guiding the team through tournaments. This showcases his growth into a dependable leader who values perseverance and collective success.