Katsutoshi Hirota is a student at Eikyo Academy and serves as the institution's ace pitcher. He is a left-handed pitcher with excellent control over his throws and also functions as a powerful left-handed hitter, often batting clean-up as the number four in the lineup. His primary role in the narrative is that of a formidable rival to Hiro Kunimi, the protagonist, as they compete within the same regional baseball district in northern Tokyo.

In terms of personality, Hirota is initially presented as a cold and calculating individual, driven by a ruthless desire to win. This manifests in a willingness to employ unsportsmanlike tactics, including spying on opposing teams, sabotage, and deliberately throwing beanballs to injure key players on rival teams. He is known for his cunning nature and a "villainous" demeanor that makes him appear older than his years. This aggressive approach to the sport aligns him closely with his high school's merit-first baseball philosophy and his coach, Shiroyama. His motivations are deeply rooted in the lessons imparted by his father, the president of a prominent construction company, who taught him never to show weakness. In one instance from his childhood, his father even advised him to carry a knife for intimidation.

Despite his antagonistic role, Hirota's character undergoes significant development. His trajectory changes when he suffers a serious elbow injury during a match, which ultimately ends his career as a pitcher. Following this, he is forced to confront the consequences of his past actions, facing retaliation from those he previously bullied. This experience, coupled with his father's death, catalyzes a profound personal transformation. On his deathbed, his father revealed that he had played the role of a villain to appear strong for his son, regretting that it led Hirota down a harmful path. This revelation allows Hirota to mature, rediscover his genuine love for baseball without his former ruthlessness, and return to the team as a first baseman and cleanup hitter.

Key relationships in his life include his family ties; he is the cousin to Fumio Ohtake and Osamu Shima, whom he initially sent to Senkawa High School as spies but who ultimately found their own purpose in baseball. His relationship with his rival, Hiro Kunimi, is central, marked by intense competition that eventually evolves into a form of mutual respect. As his personality softens, he also shows a more supportive side to his underclassmen, encouraging them in their baseball ambitions. Despite stepping back from pitching, Hirota's natural talent remains undeniable. Other characters note that his raw ability as a pitcher and a hitter rivals the very best in the series, with one character suggesting his talent as a batter might match that of another prodigy, Hideo Tachibana. Even after giving up baseball to focus on his studies, he demonstrates an exceptional capacity for concentration and intellect, quickly rising to the top of his academic class while still secretly following the summer baseball tournaments on the radio.