Hiroki Mita, a left-handed pitcher for Toushou High School, showcases exceptional talent on the baseball field. He is the older brother of Arisa Mita, who attends the same school as Otomi Tachibana and holds a rivalry with her. Hiroki possesses a serious demeanor and profound passion for baseball, actively seeking to share this passion with his initially disinterested younger sister.

During his second year, Hiroki pitched Toushou High into the quarterfinals of the National High School Baseball Tournament. In his third year, complying with Arisa's request to arrange a practice game against Meisei High School aimed at humiliating Otomi and her brothers Touma and Souichirou Tachibana, Hiroki pitched a perfect game. However, the impact of his achievement was lessened by Touma Tachibana's impressive pitching debut in that same game.

Later that third year, Hiroki led Toushou to qualify for the National Tournament (Kōshien) following a narrow, 15-inning quarterfinal victory over Meisei. The ensuing semifinal clash between Toushou and Meisei became a highly anticipated pitchers' duel featuring Hiroki against Touma Tachibana. This intense matchup extended into extra innings after nine scoreless frames, evolving into a war of attrition under pressure that highlighted Hiroki's resilience as Toushou's ace.

Hiroki's relationship with Arisa adds depth; while Arisa leveraged his baseball fame in her rivalry with Otomi, Hiroki remained focused on the game itself and persistently sought to involve Arisa in his baseball world, illustrating a protective and supportive sibling dynamic.