Mutsuki Tsuyama is a second-year student at Tsuruga Academy and a member of its mahjong team. She has dark purple hair tied into a ponytail with bangs and brown eyes, and wears the standard Tsuruga uniform with knee-high socks. Mutsuki is described as a serious and reliable person, both in her approach to the game and in her daily life. She is known for her kindness, as shown when she gives a collector's card she knows a teammate wants as a gift. She also collects the mahjong star trading card series herself.

In the story, Mutsuki is one of the players who participated in online mahjong sessions with her teammates Satomi and Yumi, which led to the discovery of another player named Momoko. As a player, Mutsuki is serious and steady, though the anime does not assign her a specific supernatural ability like some other characters. Her role on the team is that of a dependable regular player, and she is later named the next president of the mahjong club after the third-year members Satomi and Yumi graduate. Her key relationships are with her Tsuruga teammates, particularly the older members she looks up to and will eventually succeed. Mutsuki’s development is centered on her growing sense of responsibility as she prepares to lead the club, balancing her own steady gameplay with the need to support the team in the national tournament.