The character known as Kim is the Japanese name Kazuya, the twin brother of the protagonist Tatsuya. Physically identical to his brother, Kim is in most other respects his opposite. He is portrayed as a hard-working perfectionist, a diligent student, and a naturally gifted athlete, often seen as the ideal son and student. While his brother is lazy and underachieving, Kim is popular, especially with girls, and is considered a genius and the hero of their middle school. Despite his public image and his own considerable talent, Kim holds his brother in high esteem and is notably aware of Tatsuya's own potential, which often goes unrecognized by others.

Kim's primary driving force is his love for his childhood friend and neighbor, Minami, who is referred to as Debbie in some adaptations. He has made a promise to her that he will lead their baseball team to the national championship tournament at Koshien. This goal shapes his actions and defines his role in the story as the star ace of the Meisei High School baseball team. Alongside his athletic ambition, his feelings for Minami place him at the center of a love triangle with his brother, a dynamic that creates underlying tension in their relationship.

Kim shares a deep and complex bond with his twin brother, Tatsuya. Although their personalities clash publicly, Kim privately has great respect and confidence in his brother's abilities. He also shares a close friendship with a character named Boris. His relationship with Minami is one of the story's focal points; they are seen by everyone as the perfect couple, and Kim's pursuit of her is earnest and sincere. However, the narrative is brutally altered by an unforeseen tragic event, specifically a fatal accident involving Kim, which turns the lives of those around him upside down and creates a profound uncertainty about the future.

While Kim initially appears as a static character—the flawless rival and hero—his role undergoes a significant transformation. He is not just an obstacle for his brother but a catalyst for the story's central drama. His unexpected death forces his surviving brother, Tatsuya, to confront his own wasted potential and step into the role Kim once filled, carrying the weight of his brother's dream to reach Koshien. In terms of abilities, Kim is a supremely talented baseball player, particularly as a pitcher. He is depicted as a prodigy whose skill on the mound is unmatched, capable of throwing no-hit games and leading his team to victory against powerful rivals, making him the team's indispensable ace.