Mutsuki Shimazu is the mechanic for the Tokyo Metropolitan Miyake Girls' High School sidecar racing team. She works behind the scenes, preferring to support her team from the rear echelon rather than in the spotlight. Her primary responsibility is to service the sidecar machine used by the team's riders, Yuri Miyata and Megumi Meguro, ensuring it is in top racing condition. She dedicates herself to this task with intense focus, often laboring until she is drenched in sweat each day. Her personality is described as mature and reserved—she is quiet, methodical, and reliable, handling the technical demands of her role with steady competence. While she does not take part in the races directly as a rider, her mechanical expertise is vital to the team's performance, and her quiet dedication reflects a deep commitment to her work and the success of those she supports. Her notable ability lies in her hands-on mechanical skills, particularly in maintaining and tuning a racing sidecar to peak performance. As a supporting character, her role centers on providing the behind-the-scenes foundation that allows the main racing pair to compete at their best.