Mayumi Tachibana is the mother of Souichirou and Otomi Tachibana and stepmother to Touma Tachibana. She was first married to Keiichi Sawai, the biological father of Souichirou and Otomi, who passed away without disclosing his history as a Meisei High School baseball player. Following Keiichi’s death, she wed Eisuke Tachibana, Touma’s father, creating a blended household. Mayumi discovered Eisuke’s past as Keiichi’s former baseball teammate only after their marriage.

Warm, open, and naturally talkative, Mayumi readily discusses her family’s unconventional dynamics with neighbors and acquaintances. Though initially unfamiliar with baseball, she grows supportive of her children’s involvement in the sport and studies its rules through instructional books to better connect with their passion.

Her cheerful, nurturing disposition coexists with visible frustration when household rhythms falter, such as Eisuke and their friend Gorou drinking during the day and postponing family dinners. A protective streak emerges when she confronts a burglar attempting to break into her home during a pivotal baseball game. The situation resolves unexpectedly with the intervention of Haruka Ohyama’s mother, leading both women to watch the game together. During this encounter, Mayumi receives guidance on cheering etiquette, sparking a tentative friendship.

She cherishes memories of her first husband, Keiichi, though she remained unaware of his athletic background throughout their marriage. Her relationship with Eisuke reflects a contented family life built around supporting their children’s pursuits amid the intricacies of their blended household.