Koga Haruka is a central character in the series H2, acting as a primary female lead and the catalyst for the protagonist Hiro Kunimi's return to baseball. She is a student at Senkawa High School, where she serves as the manager of the school's unofficial baseball fan club. Her family includes an older brother, Fujio Koga, who later becomes the team's coach, and she comes from a household with strong ties to the sport of baseball. Her deep passion for the game is what initially drives her efforts to establish an official baseball team at her school.

Haruka is characterized by her earnestness, cheerful disposition, and deep passion for baseball. She is famously accident-prone, often tripping or dropping items, which gives her a somewhat clumsy exterior. However, this clumsiness hides a strong-willed and dedicated personality. She is sincere and forthright in her feelings, never hesitating to support her team and push them toward their goals. Her primary motivation is to see the Senkawa baseball club succeed, and she works tirelessly to give the team the legitimacy it needs to compete at the highest level, specifically at Koshien.

Haruka acts as the catalyst that reignites Hiro's passion for baseball. After a misdiagnosis forces Hiro to quit the sport, he enrolls in Senkawa High School, which lacks a baseball team. Haruka's management of the unofficial fan club and a fateful exhibition game against the school's soccer team draw Hiro back to the pitcher's mound. She is instrumental in persuading the school administration to allow an official baseball team. As the team manager, she serves as an indispensable source of logistical support and morale. Romantically, she represents the present and future for Hiro, contrasting with Hikari Amamiya, who represents his past. Her growing bond with Hiro forms a central emotional thread of the narrative.

Haruka's most significant relationship is with Hiro Kunimi. She admires his talent and becomes deeply invested in his personal and athletic journey, developing romantic feelings for him over the course of the story. Her brother, Fujio Koga, becomes the team's coach, linking her family directly to the team's fortunes. She also maintains a respectful, though occasionally complicated, dynamic with Hikari Amamiya. Her relationship with the other baseball club members is that of a dedicated and encouraging manager.

Over the course of the series, Haruka evolves from a clumsy and overly enthusiastic fan into a mature and confident young woman. Her role develops from simply managing a club in its infancy to becoming an emotional and logistical anchor for a serious competitive team. As her feelings for Hiro deepen, she provides him with a reason to look toward the future rather than dwell on the past. Her determination solidifies as she supports the team through victories and defeats, ultimately vowing to continue supporting Hiro in his professional baseball aspirations.

While notably accident-prone in her daily movements, Haruka possesses a sharp understanding of baseball fundamentals and team dynamics. Her greatest strengths lie in her unwavering dedication, emotional intelligence, and ability to motivate and comfort those around her. She diligently manages the considerable administrative duties of the baseball club alongside her responsibilities as a high school student.