Hiro Kunimi is a high school student and the central figure in the live-action adaptation of a baseball-themed story. During middle school, he was the standout player and ace pitcher of an undefeated championship team, forming a formidable trio alongside his friends Hideo Tachibana and Atsushi Noda. His path takes a sharp turn when a discredited physician falsely diagnoses him with a severe elbow condition, warning that continuing to play baseball would cause catastrophic injury. Believing his dreams of reaching the national Koshien tournament are over, Hiro enrolls at Senkawa High School, which lacks a baseball club, and initially joins the soccer team in an effort to leave the sport behind.

Hiro possesses a quiet, determined personality and a deep, if temporarily suppressed, passion for baseball. He is not quick to voice his feelings, often putting the happiness of others before his own, yet he carries a fierce competitive spirit and a strong sense of fairness. When a casual game sees him switch sides mid-match to help the school's unorganized baseball fan club against a dismissive soccer squad, his love for the game reignites. After the false diagnosis is exposed, he resolves to rebuild his baseball ambitions from scratch at Senkawa, driven by a desire to test his limits and to face his old rivals on an even field.

Within the story, Hiro is the engine behind the formation and rise of the Senkawa baseball team. He works alongside the earnest manager Haruka Koga and her brother, coach Fujio Koga, to convince the skeptical school administration to grant official team status. His relationships are central to his journey: he shares a complicated childhood bond with Hikari Amamiya, whom he once helped set up with Hideo, only to realize his own romantic feelings for her later on. With Hideo, he shares a deep respect and rivalry that pushes them both to greater heights. Haruka, who manages the club and consistently supports him, becomes increasingly important in his life as he begins to envision a future beyond high school. His friend Noda remains a loyal companion who joins him at Senkawa and stands by him through the team's struggles.

As a player, Hiro is a natural talent with elite abilities both on the mound and at bat. He throws a lively fastball with fine control and developed a sharp, high-speed slider that challenges even professional-level catchers. His forkball is less consistent, but the unpredictability of his pitches often works to his advantage, keeping hitters off balance. Beyond mechanics, his true strength lies in his unshakeable composure and adaptability during high-pressure games. Hiro’s development traces an arc from a boy who abandons a dream in despair to a young man who builds it back with his own hands, confronting false limitations, rallying a scrappy team, and eventually leading Senkawa to the national tournament. He navigates the emotional complexities of growing up, enduring loss, and managing feelings for two important people in his life, all while steadfastly aiming for a professional baseball career.