Hyuma Hoshi is the central protagonist forged by a childhood of extreme poverty and relentless discipline. He is the son of Ittetsu Hoshi, a former third baseman for the Yomiuri Giants whose own baseball career was shattered by an injury sustained during World War II. Following the early death of his mother, Hyuma and his older sister, Akiko, were raised by their embittered, alcoholic father, who imposed brutal baseball training on Hyuma from a very young age. Despite being naturally right-handed, Ittetsu forced his son to use only his left hand for all activities, employing a special training cast to build the necessary musculature to become a left-handed pitcher. This harsh upbringing initially instilled in Hyuma a deep resentment toward both baseball and his father.

Hyuma's personality is defined by an indomitable will, seriousness, and a willingness to endure tremendous physical and emotional suffering to achieve his goals. His character is marked by a profound sense of sacrifice and responsibility, often bearing burdens in silence to protect others. This is demonstrated when he accepts a false accusation to shield a friend and preserve his high school team, an act that temporarily ended his friendship with his best friend and catcher, Ban Chuta. While he initially resents his father’s grueling methods, Hyuma is driven by a deep, complex desire to live up to the immense expectations placed upon him and to prove his worth as a professional athlete. His journey transforms this initial aversion into a more nuanced understanding of his father's obsession with baseball excellence.

In the story, Hyuma’s talent begins to flourish at Seiun High School, where he forms a legendary battery with Ban Chuta. Together, they lead their underdog team to the finals of the Koshien National Championship, where his rivals include the powerful hitter Hosaku Samon and his primary adversary, the star batter Mitsuru Hanagata. After a dramatic high school career, he overcomes initial skepticism from scouts about his durability and small stature to join the Yomiuri Giants, the same team his father once played for. His professional journey is a constant battle to refine his techniques against rivals like Hanagata, now with the Hanshin Tigers, while navigating immense pressure and physical threats, including a career-endangering arm injury.

Key relationships shape his entire life. The connection with his father Ittetsu is the story's most defining and fraught dynamic, oscillating between bitter conflict, grueling tutelage, and an unspoken, shared dedication to baseball. His relationship with Ban Chuta evolves from initial hostility to an unbreakable friendship and a near-telepathic professional partnership as a pitcher-catcher battery. His rivalry with Mitsuru Hanagata is central to his development, pushing him to create ever more powerful pitches. This deep-seated competition eventually matures into mutual respect, particularly after Hanagata marries Hyuma's sister, Akiko.

Hyuma undergoes significant development, transforming from a boy who hated baseball into a man who defines himself by it. His career is marked by the constant invention and refinement of extraordinary pitching techniques. To counter the weakness of his "light throws," a byproduct of his smaller stature, he develops a series of devastating pitches. His signature weapons include a "magic fastball," the nearly unhittable "Dai League Ball" breaking ball, and the psychologically and physically dangerous "Diving Ball," which could knock batters down. The physical and emotional toll of his methods leaves lasting scars, and he grapples with his own humanity and the consequences of his unrelenting pursuit of victory. Later in his career, he even operates incognito in amateur leagues, highlighting his tactical brilliance and preference for avoiding fame while subtly influencing the sport's next generation.