Mizuki Kaji is a second-year student at Kunogi Gakuen High School and a forward on its girls' soccer team. She previously captained the Japan U-15 Women's National Team, demonstrating leadership within elite youth soccer. This background informs her approach to high school competition.

As a forward, Kaji demonstrates considerable technical skill. She maintains high expectations for discipline and respect among teammates, enforcing strict adherence to team rules and punctuality. She immediately addressed teammates Mao Tsukuda and Haruna Itō for tardiness, emphasizing collective responsibility. During matches, she actively corrects lapses in focus, such as calling out Tsukuda for defensive errors that allowed opponents to advance.

Her leadership extends beyond tactics. Kaji intervenes in conflicts, like placing a cone on Tsukuda's head to stop an argument with opponents post-match. She then apologized to the other team for Tsukuda's behavior, reflecting her view that captains must manage both performance and conduct. She expects adherence to hierarchical respect between upperclassmen and underclassmen, valuing structured teamwork over individual expression.

Teammates respect her authority, though her methods sometimes cause friction with younger players. Haruna Itō, Mao Tsukuda, and Midori Soshizaki—a former U-15 teammate now at rival Warabi Seinan—all acknowledge her leadership influence. Her reputation from the national youth team makes others hesitant to challenge her directives.

During a practice match against Warabi Seinan, resulting in a 21–0 victory for Kunogi, Kaji captained a team exclusively composed of first and second-year players. She maintained pressure on opponents and teammates throughout the game. Her actions during and after this match reinforced her consistent focus on discipline, accountability, and tactical execution.