Yamaguchi is a pretty shy person who relies on others. Though he is generally a nice person, he tends to snicker along or join in with Tsukishima whenever he’s mocking others. He is hardworking, as shown when he sought for extra training on his own, and how he continued practicing outside of the team after seeing how weak he was compared to his other teammates.
After a fatal loss in the match against Aobajōsai High, he gained a lot of confidence thanks to his teammates and Makoto Shimada. During the summer training camp, he even stood up against Kei Tsukishima, who was surprised and called him “cool” for the first time.