Tomoki Sawamura is a second-year student at Ryuumonbuchi High School and a member of its mahjong team. She possesses brown hair styled in a hime cut, brown eyes, and wears oval-shaped glasses, standing 165 cm tall. She wears Ryuumonbuchi's distinctive purple uniform.

Stoic and reserved, Tomoki speaks minimally and shows few outward emotions. She frequently uses a netbook for data analysis and prefers solitude outside mahjong activities, yet deeply values her team relationships. She occasionally displays dry humor or sarcasm, often directed at Touka Ryuumonbuchi, reflecting their familial closeness within the team. Hajime Kunihiro identifies Tomoki as the "elder sister" figure in their team's family dynamic alongside Touka and Koromo Amae.

Before joining the mahjong team, Tomoki was a highly ranked player in real-time strategy (RTS) video games. Touka Ryuumonbuchi recruited her after spending six days mastering an RTS game to challenge Tomoki directly. Serving as the team's sergeant, her analytical mahjong style relies on statistical probabilities and reviewing opponents' past records, mirroring her RTS background. Hisa Takei classifies her as a "digital" player, though this approach struggles against unpredictable beginners like Kaori Senoo.

During the prefectural tournament finals, Tomoki lost to Kaori Senoo's beginner's luck, fueling a lingering grudge. She later studied Kaori's patterns but suffered another defeat during the combined training camp arc when Kaori achieved a yakuman. In the individual tournament, Tomoki advanced to the top 20 before being eliminated after matches against Saki Miyanaga and Hisa Takei. Her involvement extended to team activities like Koromo Amae's birthday celebration in the Saki Biyori OVA, where Ryuumonbuchi solidified their familial bonds. She also advocated for repairing Nodoka Haramura's penguin plush (Etopen) during the national tournament arc, emphasizing her investment in team cohesion.