His duties primarily consist of publicity and match booking, but he also takes part in managing the finances, selling merchandise, and occasionally even assists with the training. He is a kind and friendly person, often adding ~kun to the end of the names of the Kamogawa gym members.
Yagi was originally a boxer for the Kamogawa Boxing Gym, having joined around the same time as Miyata Senior. However, despite his passion, he lacked any natural talent to survive as a pro and suffered a string of losses.
Yagi was also a master fisherman and whenever he fishes his personality changes (in the words of Miyata Senior, "he gets the face of a demon"). He is so good in fact, that it has been said that he could become the world champion of fishing. Additionally, numerous fishing ponds put up "No Yagi Allowed" signs after losing so many customers because Yagi was catching all the fish. He apparently had his fishing skills long before that, as Miyata's father learned the hard way while taking him fishing to cheer him up after a disappointing loss.
Yagi would later retire to become manager of the Kamogawa Boxing Gym.
Yagi and Kamogawa Genji, while watching a world match on the TV, talks about how they haven't raised a world champion since the Kamogawa Boxing Gym opened. Yagi informs Kamogawa that Takamura Mamoru brought a newcomer. When Kamogawa assigns Makunouchi Ippo to spar against Miyata Ichirō, Yagi watches the spar, concluding that despite Ippo's loss, he has great determination. After Ippo won the second spar against Miyata, Yagi and Kamogawa are shown how Miyata barely got hit with the Uppercut by 1cm.
When Miyata left the gym, Ippo's power from the spar two months ago isn't showing. When Kamogawa suggests to have Ippo go with Takamura, who's match is in a week, much to Yagi's dismay. Yagi and Kamogawa announces Ippo's opponent for his pro debut as Oda Yūsuke. Yagi reveals the details of Oda's fight record and fighting style to everyone.
On the day of the match, Yagi notices that Ippo's name on his trunks is "IPO" instead of "IPPO". Yagi stands at the ringside as a cornerman for Ippo's match. After Ippo's win, Yagi, along with the others, celebrates at the Sugar Ray. When Ippo has his second match against Fujiwara Yoshio, Yagi acts as a cornerman in the match.
Yagi receives the schedule for the East Japan Rookie King Tournament and shows it to the gym, revealing that Jason Ozuma is Ippo's first opponent. Yagi tells Kamogawa that Ozuma sent a sparring partner to the hospital after a hook hit him. Yagi acts as a cornerman in Ippo's match against Ozuma.
Seeing a fired up Ippo spar with Aoki Masaru and Kimura Tatsuya, he notes how Ippo is influencing the two eight rounders. When the boxers comes back from their one week training camp session, Yagi shows the poster of Takamura's JBC middleweight champion match against Yajima Yoshiaki and Ippo's match against Kobashi Kenta. Yagi begins to get excited at the thought of the gym having someone going for a Japanese title in ten years. Yagi acts as one of the cornermen in Ippo and Takamura's match. Yagi celebrates Ippo and Takamura's win with everyone.
During the Itagaki vs. Imai match, Yagi was asked by the coach to come up with a second encouragement pun after Shinoda couldn't think up another. The best he could do was pull Itagaki's Isu (Japanese for chair (or stool, in this case) away as he was returning to his corner and said Ima no ISU, Na~ISU (今のイス、ナ～イス, basically: This chair IS... nICE). Although the coach yelled at him for ruining the atmosphere, Takamura and the others found it hilarious and have been using it as a guts pose ever since.
Yagi has black short hair, a large nose, dark eye colour. He is always seen wearing eyeglasses, something he wasn't wearing in his youth.
His attire usually consists of the white KBG t-shirt and black pants. At times, he wears formal suits.
Yagi possesses a gentle and friendly personality as a manager for the gym and its boxers. In contrast to Kamogawa's gruff and strict attitude, Yagi prefers to be more nurturing and optimistic when speaking to the boxers, showing genuine concern for their well-being. That said, it sometimes allows Takamura, Aoki and Kimura to take advantage of his meeker approach (often leading to Kamogawa stepping in with a swinging cane).
Despite being quite friendly as a boxing manager, he becomes incredibly competitive and cocky when it comes to sport fishing, more than willing to make bets (and fully aware that he will win them).
Like Kamogawa, he puts a lot of trust in Ippo's ability to fight, though this is not without limits- if a bout has taken a turn for the worse and he feels that a boxer is at risk, he will readily suggest throwing in the towel.