Haruko Akagi is a first-year student at Shohoku High School and the younger sister of the basketball team's captain, Takenori Akagi. She is a kind, friendly, and sweet-natured girl with a captivating personality, but she can also be quite naive and emotionally dense, particularly when it comes to understanding the feelings of those around her. Physically, she is of below-average height for a basketball player and is notably unathletic and uncoordinated, a fact that led her to quit playing basketball after her first year of middle school despite knowing the game's fundamentals, such as layups, which she learned from her brother.

Haruko plays a pivotal role as the catalyst for the entire story. Her most significant action is approaching the delinquent Hanamichi Sakuragi after his fiftieth rejection and asking him, "Do you like basketball?". This question prompts him to join the Shohoku basketball team in an attempt to win her affection. Upon witnessing his natural athletic ability, particularly his incredible jumping power during a slam dunk attempt, she becomes convinced that he has the potential to become the team's savior. Throughout the series, she acts as a pillar of support and motivation for Sakuragi, attending his practices and games, teaching him basic skills like layups, helping him purchase basketball shoes, and comforting him after difficult losses. She is a good student and demonstrates her loyalty and sense of responsibility by tutoring Sakuragi and his teammates to prevent them from being academically disqualified from the national tournament. She is also not afraid to defend Sakuragi from criticism, though she briefly and unfairly turns against him after witnessing a fight, an error for which she later apologizes.

Her key relationships are central to her character. She has a respectful and supportive relationship with her brother, Takenori, for whom she consistently cheers. HEr feelings for Sakuragi are more complex; while she considers him her best friend and is always there for him, she remains completely oblivious to his deep romantic love for her throughout most of the series. This is largely due to her own unrequited crush on Sakuragi's rival, the aloof and talented rookie Kaede Rukawa. Like many other girls at school, she is infatuated with Rukawa, but his single-minded focus on basketball means he never acknowledges her feelings. Over time, she begins to realize the futility of her crush on Rukawa as she sees he has no room in his life for romance.

Haruko undergoes subtle but meaningful development. Her emotional journey culminates during the national championship match against Sannoh, when Sakuragi suffers a serious back injury. In a pivotal moment, he confesses to her that he truly loves basketball, a statement that serves as an indirect declaration of his feelings for her. She is left stunned and blushing by this confession and later cries tears of joy after he makes the game-winning shot. In the epilogue, after the tournament, she takes on the role of the team's second manager. Her ongoing care is shown through her weekly correspondence with Sakuragi while he undergoes rehabilitation for his back injury. In her letters, she updates him on the team and expresses her eager anticipation for his return, signing off with phrases that suggest a potential shift in her own feelings toward him, such as "I'll be waiting for you".