Tadashi Yamaguchi attends Karasuno High School and plays on its boys' volleyball team. Bullied in elementary school for his freckles and slender build, he formed a lasting admiration and friendship for Kei Tsukishima after Tsukishima unintentionally intervened. This bond led Yamaguchi to join the volleyball club alongside Tsukishima in junior high and later at Karasuno, perceiving volleyball as less intimidating than soccer or basketball. Physically, Yamaguchi possesses dark green hair with a distinctive flyaway strand, freckles, amber eyes, and a thin build. His height increased from 179.5 cm to 180 cm during high school, and by his third year, he adopted a shorter hairstyle. Initially shy, self-doubting, and prone to nervousness, especially under pressure, Yamaguchi often aligned with Tsukishima, joining in teasing teammates while fiercely defending Tsukishima. Despite his reserved nature, he demonstrated strong loyalty and work ethic. Being the only first-year not in Karasuno's starting lineup fueled his insecurity and motivated him to seek extra training. His volleyball development focused on mastering the jump float serve, a technique learned from Karasuno alumnus Makoto Shimada after witnessing its effectiveness. This unpredictable serve became his specialized skill as a pinch server and backup middle blocker. His first official match serve against Aoba Johsai failed due to overwhelming nerves, strengthening his resolve. He later articulated to Tsukishima that "pride" was sufficient motivation to persevere. Subsequent training improved his serve’s accuracy, allowing him to target weaknesses or land near boundaries, with a successful serve in a rematch against Aoba Johsai marking a significant confidence milestone. At a training camp, Yamaguchi confronted Tsukishima about his apathy toward volleyball, urging full commitment. This demonstrated Yamaguchi’s growing assertiveness. By his third year, he was appointed team captain, a role attributed to his empathy, resilience, and ability to support teammates—traits deemed more suitable for leadership than Hinata or Kageyama’s competitiveness. After graduating high school, Yamaguchi attended college and planned to work for a home electronics company in Miyagi Prefecture.