Sōichirō Tachibana, stepbrother to Toma and biological brother to Otomi, navigates a blended family dynamic shaped by their parents’ remarriage. Born merely ten minutes before Toma, they share not only a birthday but a tangled bond blending camaraderie and rivalry. His choice to cede the pitcher position to Toma—settled by a childhood game of rock-paper-scissors—puzzles their stepfather Eisuke and coach Gorou, particularly given the legacy of Sōichirō’s late father, a once-celebrated pitcher whose career succumbed to injury. Privately, he rationalizes this sacrifice, believing Toma thrives with a dependable catcher like himself, while he would lack equivalent support in reverse.

A natural athlete and tactician, Sōichirō commands the field as catcher, his analytical mind orchestrating team strategy. Protective of Otomi, he frequently nudges Toma to ensure her safety, reinforcing their intricate sibling dynamic. Charismatic and once notorious for a string of fleeting middle-school romances, his focus shifts upon developing unrequited feelings for Haruka Ohyama, the baseball team’s manager. Though she remains indifferent, he steadily pursues her, frequenting her workplace—a quiet evolution from his earlier flirtatious persona.

Fragmented memories of his father, Keiichi Sawai, quietly shape his approach to baseball, a subject he seldom vocalizes. The family’s ties to Meisei High’s faded baseball glory fuel his dedication, aligning with Toma’s drive to resurrect the school’s reputation. His bond with Haruka intertwines with unresolved childhood threads, as she shares a pivotal past encounter with Toma—a subplot where Sōichirō occupies a periphery role.

His arc balances personal introspection with team leadership, prioritizing collective ambition over individual acclaim. Strategic brilliance and unwavering loyalty anchor his growth, while subtle narrative undercurrents probe his internal tensions—grappling with paternal legacy and self-identity within the sport—without overt exposition.