An average looking boy with an average build. Yamaguchi has short black hair, often portrayed as brown or even green, with a piece of hair sticking up at the top. He is also well recognized for his freckles. Back in middle school, he was teased for his freckles as other kids called his face "pimply".
He's a pretty shy person who relies on others. After a fatal loss in the match against Aobajousai, he gained a lot of confidence thanks to his teammates and Shimada Makoto. In the Summer training camp he even stood up against Tsukishima, who was surprised and called him "cool" for the first time.