Kōshi Sugawara served as the vice-captain and setter for the Karasuno High School volleyball team during his third year. He joined Karasuno alongside junior high teammates Daichi Sawamura and Asahi Azumane, motivated by the school's legacy in volleyball. His first two years, however, saw the team struggle with early tournament exits and limited resources. Standing at 174.3 cm with light grey hair, hazel-brown eyes, and a distinctive mole under his left eye, Sugawara possessed a slender build. His appearance remained consistent after the timeskip, with slightly longer hair. Sugawara displayed a gentle, supportive nature, acting as a stabilizing force for teammates. He also exhibited a mischievous streak, encouraging Shoyo Hinata to crash an uninvited training camp. Occasional pride surfaced, such as claiming credit for naming the "Back Minus" technique and demanding recognition from Tobio Kageyama. During intense matches, he sometimes revealed frustration, including visible pouting after losing points. Initially Karasuno's starting setter, Sugawara moved to a substitute role after Kageyama's arrival, acknowledging the first-year's superior technical skill. Despite moments of jealousy, he prioritized team success over personal ambition. Determined to avoid future regrets, he continued club activities despite most third-years retiring, influencing Daichi, Asahi, and Kiyoko Shimizu to remain. Off the court, Sugawara proved strategically vital. He devised tactics like synchronized attack hand signals overnight and created verbal cues ("bring it," "send it to me") to improve coordination between Kageyama and Hinata. His sharp observation identified opponents' weaknesses, such as targeting aggressive spikers with precision float serves to disrupt their rhythm. His limited match time as a pinch server and occasional setter became an advantage, as opponents lacked data on his patterns. He developed specialized techniques, including the "Calculated One-Point Two-Setter" strategy against Shiratorizawa, where he set to Kageyama acting as a spiker, expanding Karasuno's offense. His physical abilities included a 299 cm spike reach and 285 cm block reach. Post-graduation, Sugawara became an elementary school teacher in Miyagi Prefecture by 2018. In 2021, he maintained connections with former teammates, attending an Olympic match between Japan and Argentina alongside Daichi, Ryūnosuke Tanaka, and Kiyoko Shimizu. His favorite food was super spicy mapo tofu, and he expressed lighthearted concern about being shorter than many underclassmen. His name derived from "supporting one's elders" (Kōshi) and "sedge plain" (Sugawara).