Tsugen Sasaki is a second-year student at Doshisha University and plays rugby for the university team, wearing the number 7 jersey in the forward position as a right flanker. He possesses short black hair, green eyes, and a medium build.

His personality exhibits a spiritual nature, demonstrated when he guides teammates Seiichirou Shingyouji and Pongsaklek Yunu through meditation exercises in Episode 7 to improve their focus. On an earlier occasion, when Pongsaklek Yunu expressed curiosity about a conversation between Natsusa Yuzuki and Sunao Konoe concerning Ibuki Ueoka, Sasaki advised Yunu that understanding their Japanese discussion was unnecessary.

His given name, Tsugen, combines the characters for "traffic, pass through, avenue, commute" (通) and "string, bowstring, handle" (弦). His surname, Sasaki, incorporates the meanings "help, aid" (佐) and "tree, wood" (木).