Tatsuya Uesugi is the elder fraternal twin brother of Kazuya Uesugi. He cultivates an image of laziness and indifference, deliberately stepping back so his naturally less gifted but diligent younger brother can shine in academics and baseball, particularly as their middle school's ace pitcher. This stems from Tatsuya's profound altruism and aversion to competition, especially against Kazuya, whom he deeply cares for despite their differences. Both brothers harbor lifelong affection for their childhood friend and neighbor, Minami Asakura. Tatsuya consciously suppresses his own feelings for Minami, publicly endorsing the assumption of her destined future with Kazuya, though this inflicts significant emotional pain.

Kazuya's death in a traffic accident on the morning of a crucial regional tournament shatters Tatsuya's world, forcing him to confront his potential and responsibilities. Inheriting his brother's position as Meisei High School's ace pitcher, Tatsuya steps onto the mound despite his previous avoidance of serious commitment. His raw, natural athletic talent, always superior to Kazuya's but underdeveloped due to reluctance, fully emerges under this pressure. He dedicates himself to fulfilling Kazuya's promise to take Minami to the national tournament at Koshien Stadium, transforming from a passive observer into a determined athlete fueled by responsibility and love.

Tatsuya's personality blends apparent indifference with deep sensitivity. Outwardly easygoing, troublesome, and blunt—often perceived as selfish or lazy—he possesses strong innate kindness and dislikes causing hurt or conflict. He is deeply thoughtful, prone to overthinking, and vulnerable to external pressure. His relationships, particularly with Minami and teammates like catcher Kotaro Matsudaira, evolve from initial friction into deep bonds forged through mutual respect and shared goals. His journey involves significant growth as he learns to harness his talent for others while grappling with guilt, loss, and his own desires.

Later, in "Miss Lonely Yesterday," Tatsuya attends Meisei University on a baseball scholarship while Minami studies elsewhere. Their physical separation strains the relationship, revealing lingering communication issues despite their strong bond. Tatsuya immerses himself in his university baseball career, relentlessly pursuing excellence on the mound. The story culminates during a pivotal game where Minami, sensing his struggle, travels to watch him play. Witnessing his determination reignites her feelings. They reunite emotionally after the game, marking a mature reconciliation and a firmer commitment to their future, demonstrating Tatsuya's continued development in confronting challenges and expressing his commitment.