Aoba Tsukishima is the third of four daughters in the Tsukishima family, which runs a batting center and cafe. From a very young age, she demonstrated an extraordinary passion and talent for baseball, a pursuit encouraged by her father. As a girl, she is barred from participating in official tournament games, a fundamental limitation that shapes much of her journey and her interactions with others. Her mother died when she was young, and she was particularly close to her older sister, Wakaba, whose sudden death in a drowning accident during elementary school is a defining tragedy in Aoba's life.

In her youth, Aoba is portrayed as a classic tomboy with a sharp tongue and a quick temper, especially towards Kou Kitamura, a boy her sister's age who lives next door. Her animosity towards Kou stems from intense childhood jealousy; she resented how much of Wakaba's attention he commanded and felt she had to compete with him for her beloved sister's affection. This manifests in constant bickering and a public declaration of hatred for Kou that lasts for years. Despite her hostile exterior, she is an immensely dedicated and hardworking athlete who maintains a rigorous personal training regimen. She is a natural prodigy on the mound, possessing a flawless and smooth pitching form that Kou secretly studies and eventually adopts as his own. Her fastball is remarkably powerful, and her understanding of pitching mechanics and strategy is highly advanced.

As the story progresses into their high school years, Aoba's role evolves significantly. While she continues to verbally spar with Kou, she becomes his most important and critical coach. She is one of the few people who can give him an honest, unsentimental assessment of his skills, teaching him breaking balls and refining his technique. Her motivation is complex; on the surface, she wants to see the Seishu baseball team succeed, but on a deeper level, she begins to channel her own thwarted ambitions through Kou. As the star first baseman Yuuhei Azuma points out, Aoba is using Kou's body as a proxy to reach the national Koshien tournament, a dream she cannot achieve herself because of her gender. This unspoken pact becomes a core part of her character, fueling her dedication despite her claims of indifference.

Aoba's key relationships define her personal development. Her bond with her late sister Wakaba is the emotional center of her past; she idolized Wakaba and measures herself against her memory. Her relationship with Kou is the most dynamic, slowly transforming from genuine childhood resentment to a deep, unspoken understanding and mutual respect. They understand each other's grief and dedication better than anyone else, even as they refuse to admit it. The arrival of Akane Takigawa, a girl who bears a striking resemblance to Wakaba, creates new emotional turmoil for Aoba. While she becomes close friends with Akane, she also struggles with jealousy, fearing that Kou will transfer his feelings for Wakaba onto Akane, thereby solidifying a relationship that would close the door on her own hidden emotions.

Throughout the series, Aoba's growth is marked by her gradual acknowledgment of her true feelings. Her pride and the lingering shadow of Wakaba's request not to "take Kou away" force her to bury her affection under a layer of sarcasm and denial. Her abilities on the field are showcased in practice games and an exhibition match for a women's college team, where she dominates with her pitching and confirms that her talent is of a national caliber. However, she ultimately turns down a path that would lead to the women's national team, choosing instead to stay and help guide Kou and Seishu to Koshien.

By the story's climax, Aoba's carefully constructed walls finally break down. After Kou makes a cryptic confession that he loves her in the only way he knows how, promising to throw a 160 km/h fastball and take the team to Koshien, she is left in turmoil. When he fulfills that promise in the final game, she is overwhelmed by the truth of his words. In the series' emotional resolution, she drops her hostile facade, crying in Kou's arms while still insisting she hates him, a final affirmation of her tsundere nature. In the epilogue, she is seen more openly accepting his affection, holding his hand and finally admitting to herself that he is the only person who truly shared her pain and understood her, even as she playfully insists she cannot stand him. She remains, to the end, a fierce and talented individual whose love for baseball is matched only by her deeply buried capacity for love itself.