Fujio Koga serves as the head coach of the Senkawa High School baseball team. He is the older brother of Haruka Koga, and there is a significant age gap between the siblings. Coming from a wealthy family, Koga was unemployed after graduating from college, spending his time traveling, until the opportunity to coach at Senkawa High School arose. He had experience playing baseball in high school, but no prior coaching experience.

His decision to become a coach was almost accidental. The principal of Senkawa High School had promised to establish a baseball team and, while drinking tea at a café, witnessed Koga becoming passionately angry while watching a high school baseball broadcast on television. The principal was impressed by Koga's conviction that student baseball should be evaluated on effort and education, not just results, and offered him the position.

Koga's personality is defined by a deep-seated belief that high school baseball is fundamentally a part of education. He is a garrulous and somewhat eccentric figure who often makes plans without thinking, chats freely with his players, and operates on his own sense of time, giving him a comedic presence. However, beneath this unorthodox exterior lies a sharp baseball mind. He has demonstrated a remarkable ability to predict the flow and outcome of a game, sometimes providing instructions that perfectly anticipate the opponent's actions. Despite his own strategic insight, he is notably free of ego and is willing to accept tactical advice from his own high school players, such as Hiro and Noda. This flexibility and rejection of personal ambition for the sake of winning are central to his coaching philosophy, as he believes a coach should never steal a player's dreams or freedom for the sake of victory.

His motivations are largely shaped by his own painful past as a player. He harbors a deep grudge against his former high school coach, Coach Shiroyama of Eikyo High School. Despite possessing excellent baseball skills, Koga was relegated to the bench during his own high school years because of Shiroyama's ruthless, win-at-all-costs methods. After this experience, he stepped away from baseball entirely and wandered for a time. This background explains his fierce opposition to coaches who prioritize winning over the educational welfare of their students. His primary motivation as a coach is not merely to win, but to build a team and a system that respects the spirit of high school baseball, and ultimately, to defeat Coach Shiroyama.

His key relationships revolve around two figures. One is his sister, Haruka, who observes his actions and provides a link to his personal life. The other, more crucial relationship, is with Coach Shiroyama. Their conflict comes to a head in the regional semi-finals. Koga, who knows Shiroyama's strategy intimately, systematically dismantles his former coach's game plan. During the match, Shiroyama finally recognizes his former player, and Koga's team ultimately wins the game, resolving his long-held grudge. The defeat is a major shock to Shiroyama, who is forced to retire.

As a coach, Koga shows notable development. He starts as an unemployed wanderer with a painful past, but transforms into a dedicated and effective leader. His abilities are not conventional, but they are highly effective. He possesses a unique talent for strategic foresight and creating a team environment where players are motivated and free to grow. He has no prior coaching experience, but through a combination of innate knowledge, a willingness to learn from his own players, and a powerful commitment to his educational principles, he proves to be an exceptional manager for a high school baseball team.