Yagi is slender with straight, light brown hair and dark grey eyes. He is good-looking according to the girls at the academy and is generally seen smiling. He also has nosebleeds whenever he sees the children in the daycare room.
He is good in his studies and soft-spoken with everyone he associates with, especially his best friend, Nezu, and the children in the daycare room. While his reaction of having a nosebleed upon seeing them disgusts Inomata Maria, it is simply out of an innocent fondness for children owing to his difficult childhood. He is also thoughtful, sometimes buying gifts for the daycare kids.
He is also extremely popular with girls, with a crowd of them around him at the lost-and-found booth of a matsuri festival once which made Hayato comment if it was a "host club" he was managing.
Yagi is the younger child of two. He has an older sister, Kotone and parents who are never seen. Yagi spent a lonely childhood. His overall cheerful demeanor hides the fact that his parents' frequent work-related absences and his sister's busyness make him very much alone all the time.
Nezu Chuukichi is his closest friend who spent time with him frequently during his childhood. On one occasion, Yagi's parents missed his birthday which made him cry really hard. Upon seeing that, Nezu invited him over to his house to celebrate his birthday with his large family. Nezu's mother comforted him, and as he started crying again, this made the toddler twins, Nezu Suekichi and Kichi cry too. Nezu promptly encourages Yagi to stop doing so to prevent his brothers from crying. He agreed and this brought smiles on the twins faces which made Yagi fall in love with babies. This lifted his spirits and he was able to experience what a loving home felt like.
When Ryuuichi and Kotaro was picking up a sad Kirin from her home in the morning as her mother was unable to do so herself, he encounters Yagi and the twin brothers, Kichi and Suekichi. Fearing that Yagi as a "pervert" will harass Kotaro, the twins ran away with Kotaro to "save the baby", leaving behind a nonplussed Ryuuichi. Then, Ryuuichi leaves Kirin in the arms of Yagi to chase after them. Yagi confessed to Kirin after awhile that he felt lonely as a child and was glad to be able to play with them in the daycare room. He laughed at his own words, claiming them as a joke, but Kirin saw that as an honest side of Yagi and invited the "sparkly onii-chan" to come play with them as often as he likes. Later on, Yagi brought a lot of his chocolate to share with the daycare children and said that it was coincidentally his birthday as well. The children drew a "ticket to visit them anytime" which was presented by Kirin to him, which made him very happy inside.
- Yagi in his name means "mountain goat".