Mayumi Shinjo serves as the older sister to protagonist Dome Shinjo. Their deceased father was a renowned Yomiuri Giants baseball player, leaving a legacy in the sport. After his death, Mayumi and their mother actively discouraged Dome from pursuing baseball, fearing the risks and pressures of the career that claimed their father, sparking recurring familial tension.

Within the family, Mayumi took on significant domestic duties, managing household responsibilities and supporting their mother, Akane Shinjo. This role established her alongside Akane as a figure of authority regarding Dome's activities and aspirations.

Mayumi's opposition to Dome's baseball career gradually shifted. Witnessing his unwavering passion, inherited talent, and deepening commitment to the Giants prompted her towards a more supportive stance. Her backing became evident during Dome's difficult challenges, notably against the veteran Dragons captain who first hit his signature pitch, and later with the arrival of the skilled Spanish player, Don Carlos, who harbored a personal rivalry with their late father, heightening the stakes for Dome.

A key part of Mayumi's character arc involved reconciling with Dome over his baseball path. While retaining her protective instincts as an older sister, her initial discouragement transformed into reluctant acceptance and eventual support. This change stemmed from recognizing Dome's genuine love for baseball and his determination to honor their father's legacy through his own achievements.