Yamaguchi is recognizable by the freckles sprinkled across his cheeks. His hair is short and black, and he has thick eyebrows. His eyes are a dark indigo color.
Yamaguchi is rather quiet, enduring, and friendly. He might look easily irritated, but it takes a lot to actually get him mad. He isn't the strongest, but he has a strong will.
Despite initially being a bit of a pushover, as a third year he gains the confidence to stand up against Midousuji and act as his conscious (despite he himself saying he's bad at it), and is also quick to see through the deceptive plans of others, revealing himself as an insightful and headstrong character when he refuses to fall into their traps.
Despite disagreeing with Midousuji at times, Yamaguchi sticks to his orders and is willing to go exhaust himself completely to follow through with them.
Skills and Techniques
Initially, as a second year sprinter, Yamaguchi hadn't demonstrated any noteworthy cycling abilities. Ishigaki, however, praised him for his endurance on the flats, despite his trouble on the climb.
As a third year, he is noted by Funatsu to be excellent at pulling, exercising a solid sense of stability while doing so. This strength is also noted to be somewhat of a hidden ability, as Funatsu also states he's never seen it before. He compares Yamaguchi's sudden development as a sprinter to a wild cherry blossom quietly coming into bloom during spring.