Koshi Sugawara is a third-year student at Karasuno High School and serves as the vice-captain of the boys' volleyball team. He was the team's original starting setter before the arrival of the prodigious first-year Tobio Kageyama, a transition that fundamentally redefined his role on the team. Unlike many of his peers in their final year, Sugawara chose to continue playing club volleyball rather than retiring to focus on academics, driven by his deep desire to avoid future regrets.

Sugawara is characterized by his gentle, supportive, and perceptive nature. He possesses a remarkably calming presence that steadies his teammates during high-pressure moments, earning him a reputation as the emotional pillar of Karasuno. His strong empathy allows him to intuitively understand and manage the dynamics within the team, making him a natural caretaker who is often regarded as a parental figure to the younger members. He is also highly intelligent and strategically minded, capable of devising effective tactics from the bench.

His primary motivation is contributing to the team's success, regardless of his personal playing time. Sugawara demonstrates profound emotional maturity by wholeheartedly supporting Kageyama, the player who took his starting position, because he firmly prioritizes the team's collective victory over individual glory. His core philosophy revolves around the idea that success in volleyball is dependent on the entire team working together.

As a substitute setter and pinch server, Sugawara plays a crucial role that extends far beyond his minutes on the court. He serves as the team's primary morale booster and emotional anchor. He bridges gaps between teammates and offers vital tactical advice from the sideline. His strategic mind comes to the forefront when he suggests the verbal cues for Kageyama and Hinata's quick attack, demonstrating his selflessness and tactical insight.

His relationship with captain Daichi Sawamura is one of deep mutual respect and complementary leadership; Sugawara strongly supports Daichi's authority while offering his own steady perspective. He shares a close friendship with the timid Asahi Azumane, often acting as a link between him and the rest of the team. His most defining relationship is with Tobio Kageyama. Instead of resenting the first-year, Sugawara becomes one of Kageyama's most important supporters, offering consistent encouragement and helping him integrate into the team's social fabric. He also watches over the enthusiastic first-years with a warm and guiding presence.

Sugawara's character arc is one of graceful adaptation and discovering value beyond the spotlight. He transitions from a starting player to a bench player without bitterness or jealousy. His journey is not about regaining a starting position but about redefining what it truly means to be a valuable member of a team. He learns that leadership, emotional support, and selfless contribution are just as vital to a team's success as raw technical skill. He proves that a player's significance is not measured solely by their statistics or starting status.

As a setter, Sugawara is rated highly in intellect and technique. He excels at reading the flow of the game, anticipating opponent strategies, and devising effective counter-plays. His serves are reliable and often used to break the opponent's rhythm, making him a capable and dangerous pinch server. While his physical attributes such as power, jumping, and speed are average, he fully compensates for these limitations with his sharp tactical awareness and deep understanding of team dynamics. His greatest ability is his emotional intelligence, which allows him to elevate the performance and composure of everyone around him.