Fujio Koga is the older brother of Haruka Koga and serves as the head coach of the Senkawa High School baseball team. There is a notable age gap between the siblings. Before becoming the coach, he was a college graduate from a financially comfortable family who remained unemployed for a period, traveling extensively, and later worked as a driver for the family business.

His appointment as coach was unexpected. While watching a Koshien baseball game on a portable television in a coffee shop, his excited and insightful commentary was overheard by Senkawa's principal, Michio Yanagi. The principal was impressed and promptly offered him the position of head coach for the newly established club.

Fujio Koga appears as a laid-back and somewhat eccentric figure, with a beard and sunglasses as his trademarks. He has a fondness for collecting promotional freebies and generally carries a relaxed demeanor. Despite this casual exterior, he possesses a deep-seated passion for baseball and a sharp analytical mind for the sport. His coaching philosophy is largely shaped by his own high school experience as a player under Coach Shiroyama at Eikyo High School. When he rebelled against Shiroyama's ruthless, win-at-all-costs approach, he was punished by being kept on the team’s roster for three years without ever being allowed to play in a single official game. This personal history motivates him to foster a team environment built on genuine love for the game, camaraderie, and sportsmanship, directly opposing the methods of his former coach.

In the story, he serves as a mentor and a source of strategic guidance for the Senkawa team. He does not impose strict, hands-on training routines but instead trusts the instincts of his players, particularly the core battery of Hiro Kunimi and Atsushi Noda, allowing them to develop their talents naturally. He provides crucial tactical advice during games and maintains a steadying influence on the team's morale. His leadership style is unorthodox but effective.

His relationship with his sister Haruka is one of mutual support, as he takes on the coaching role for the club she founded. His connection with the principal is built on a shared respect for baseball analysis. The most significant personal challenge and development for Fujio comes when Senkawa faces Eikyo, turning the match into a chance for symbolic redemption from his past mistreatment. Over the course of the series, he grows from an aimless wanderer into a committed and invested leader, finding purpose in guiding the team toward their goals. While he does not possess flashy physical abilities as a player, his notable strengths lie in his strategic intelligence, his ability to recognize and nurture talent, and his quiet yet firm leadership.