Koshi Sugawara attended Nagamushi Junior High before enrolling at Karasuno High School alongside Daichi Sawamura and Asahi Azumane. The trio joined Karasuno's volleyball club anticipating national tournaments due to the school's legacy, but encountered a declining team without a coach and few practice matches. After a second-round elimination in their first-year Miyagi Prefecture Interhigh Qualifiers, Sugawara remained under Daichi's leadership. As third-year vice-captain, Sugawara initially held Karasuno's starting setter position until first-year prodigy Tobio Kageyama demonstrated superior setting. Prioritizing team advancement, he voluntarily relinquished his role while committing to skill development. Sugawara has light grey hair, hazel-brown eyes, and a distinctive mole under his left eye. Standing at 174.3 cm during his third year with a slender build, his gentle smile enhanced his approachability. Post-timeskip, he grew to 174.6 cm with slightly longer hair. By 2021, he reached 176.2 cm and 68.6 kg. His perceptive, intelligent nature facilitated emotional support and strategic innovations. He created hand signals overnight and verbal cues like "bring it" to coordinate Kageyama and Hinata. He detected subtle teammate behavioral shifts, like Tanaka's atypical gym key request. Though typically calming, he occasionally displayed mischief—encouraging Hinata's unauthorized camp attendance—and rare pride, demanding Kageyama credit him as a mentor and claiming naming rights for the "Back Minus" technique. High-pressure matches like Shiratorizawa revealed competitive frustration. His volleyball skills emphasized tactical intelligence over physicality, reflected in 4/5 game sense and technique versus 2/5 ratings in power, jumping, stamina, and speed. Sideline observation allowed opponent pattern analysis, while his unpredictable substitutions exploited sparse opponent data. He mastered synchronized attacks during summer camp and practiced spiking to diversify offense. Critical matches featured specialized tactics like float serves targeting Kentarō Kyōtani and his "Calculated One-Point Two-Setter" strategy creating five attack options. He facilitated early Kageyama-Hinata cooperation by mediating disputes and suggesting tactics during their 3-on-3 match. Against Aoba Johsai, he intervened when Kageyama criticized Hinata and identified Oikawa Tooru's serving strategy. His leadership enforced discipline during away matches while providing emotional support after losses. Post-graduation, Sugawara became an elementary school teacher in Miyagi. By 2018, he was established in education. In 2021, he attended an Argentina-Japan Olympic volleyball match with Daichi Sawamura, Ryunosuke Tanaka, and Kiyoko Shimizu, receiving photographic confirmation of Asahi Azumane and Yu Nishinoya traveling through Egypt. He enrolled in college preparatory classes alongside Daichi. His family includes one younger brother, and his favorite food is super spicy mapo tofu. His surname inspiration originated from a creator's workplace senior nicknamed "Suga-san."