Koshi Sugawara is a third-year student at Karasuno High School and serves as the vice captain of the boys' volleyball team. He plays as a setter and also acts as a pinch server. Sugawara was the team's starting setter before the arrival of the first-year prodigy Tobio Kageyama, and he eventually transitions into a backup role while continuing to contribute significantly to the team's success.

Sugawara is known for his calm, kind, and supportive personality. He has a soothing presence that helps stabilize his teammates during high-pressure matches, and he often acts as a pillar of emotional support for the team. His perceptiveness allows him to read the mood of the group and notice subtle details, such as when a teammate is hiding something. He is also highly intelligent and tactically minded, able to devise effective strategies during games, such as the verbal cues he creates for Kageyama and Hinata's quick attack. His dedication to volleyball is strong; despite no longer being the starting setter, he chooses to continue club activities when most third-year students retire to focus on academic futures, explaining that he does not want to have regrets.

His primary motivation is to support the team in any way he can and to avoid leaving anything undone. He firmly believes that success in volleyball requires collective effort, expressed through his attitude that no one wins alone. This perspective helps him accept his reduced playing time gracefully and instead focus on boosting team morale and helping younger players develop. His encouragement and positivity are crucial in maintaining the team's unity and fighting spirit.

In the story, Sugawara's role is that of a reliable upperclassman and a steadying influence. He works alongside captain Daichi Sawamura to lead the team, offering both tactical advice and emotional reassurance. His relationship with Daichi is built on mutual respect, and he supports Daichi's leadership without seeking the spotlight. He becomes a mentor figure for Kageyama, choosing to support the first-year setter rather than resent him, which helps the team integrate the new talent. His interactions with the other members, such as Asahi Azumane and the younger players, are marked by warmth and a sense of responsibility.

His personal development centers on embracing his new role. Initially, he experiences disappointment at losing the starting position, but he grows to understand that meaningful participation does not require being on the court at all times. He discovers fulfillment in being a strategic asset and a source of stability for his teammates. This maturity is reflected in his calm leadership, his willingness to execute specific plays when called upon, and his constant encouragement, which inspires trust and cohesion within the squad.

As a player, Sugawara's strengths lie in his game sense and technique. His official ability parameters rate his intellect and technique at 4 out of 5, while his power, jumping, stamina, and speed are lower. He compensates for his average physical attributes with clever tactics, accurate tosses, and a deep understanding of the flow of a match. He is not a flashy player, but he reliably runs the offense when he is on the court and can come in as a server to turn the momentum of a set. His ability to rally his teammates with a few words or a calm look makes him an invaluable part of the team, even when he is not the one setting the ball.