Haruhiko Yagi is a recurring character who serves as the manager of the Kamogawa Boxing Gym. He is also a contemporary of Miyata Senior, having joined the gym around the same time as the elder Miyata.

Yagi originally started as a boxer at the Kamogawa gym, driven by a strong passion for the sport. However, he lacked the natural talent needed to succeed as a professional and suffered a string of losses, which ultimately led him to retire from active competition. Following his retirement, he took on the role of gym manager, where his duties are extensive. He is primarily responsible for publicity and match booking, but also manages finances, sells merchandise, and occasionally assists with training sessions.

In terms of personality, Yagi is known for being kind and friendly, often addressing the younger gym members with the honorific "kun," which reflects his gentle and approachable nature. This nurturing and optimistic demeanor stands in contrast to the gruff and strict attitude of Coach Kamogawa. Yagi shows genuine concern for the well-being of the boxers, though his meeker approach sometimes allows fighters like Mamoru Takamura, Masaru Aoki, and Tatsuya Kimura to take advantage, which often leads to Kamogawa stepping in to restore order. While he is usually quite gentle, Yagi possesses a remarkable and starkly contrasting side to his personality when he engages in his hobby of fishing. In the words of Miyata Senior, "he gets the face of a demon". During fishing, he becomes incredibly competitive and cocky, fully confident in his abilities and more than willing to make bets.

Yagi's role in the story is primarily a supportive one, working behind the scenes to ensure the gym runs smoothly. He often stands at ringside with Coach Kamogawa during matches, acting as a cornerman for boxers such as Ippo Makunouchi and Takamura. He is shown putting a great deal of trust in Ippo's ability to fight, but this trust has limits. If a bout takes a dangerous turn and he feels a boxer is at significant risk, Yagi is readily prepared to suggest throwing in the towel. His relationship with the gym members is generally warm, but one of his most notable connections is with Miyata Ichiro. When Miyata Senior was hospitalized after a loss, Yagi visited and was deeply affected by the determined look on the young Ichiro's face, a moment he would never forget.

Yagi also experiences moments of development through his unique talent as a master fisherman. His skill is so extraordinary that it has been said he could become the world champion of fishing, and numerous fishing ponds have reportedly put up "No Yagi Allowed" signs because he was catching all the fish. This skill becomes a recurring source of humor and interaction. During a fishing trip with the main cast, he and another trainer won a fishing competition by a landslide, which prompted Takamura to train in fishing specifically to challenge him to a rematch. The rivalry culminated in a contest at a pond that still welcomed Yagi, though the result was so one-sided that the pond subsequently banned both of them.

Yagi's most notable ability, aside from his managerial skills, is his preternatural talent for fishing, which transforms his typically gentle personality into something far more intense and competitive. His physical appearance is consistent throughout the series; he has short black hair, a large nose, dark-colored eyes, and is almost always seen wearing glasses.