Haruka Ōyama, daughter of Meisei High's baseball coach Gorou Ōyama, relocated to the school with her father after junior high. Initially excelling in rhythmic gymnastics, a fall from an old referee stand while gathering baseball data resulted in a broken arm, ending her gymnastics pursuits and leading her to become the baseball team's dedicated trainer and manager.

Kind, supportive, and openly vocal, Haruka possesses deep baseball knowledge and a notable strategic mind, often focusing on future games with an opportunistic and optimistic outlook. Her grasp of the game rivals her father's, making her invaluable for scouting opponents and developing insights. She deeply cares for those around her.

Haruka fondly recalls meeting Touma Tachibana at age three during his mother's funeral, where she poured soda on him for disruptive behavior, an event he doesn't remember but she considers their childhood friendship. She later took him on a walk to revisit the site, hoping to trigger his memory without success.

Her interactions with the Tachibana brothers differ: she frequently clashes with Souichirō over his short-term tactics versus her long-term planning, while her relationship with Touma remains generally harmonious. Though both brothers develop romantic feelings, Haruka maintains a platonic friendship with them. She playfully deflected Otomi Tachibana's question about preferring one brother.

Haruka enjoys solving puzzles, becoming intensely focused until finding an answer, a trait that once prompted her spontaneous, lengthy walk with Touma. As the baseball team's manager, her holistic dedication drives her to perform every necessary task to foster the team's development and success.