Yamaguchi is a pretty shy person who relies on others early on, most likely stemmed from being bullied because of his appearance. Despite this, Yamaguchi is generally a nice person but does tend to join Tsukishima in teasing others, mainly Kageyama and Hinata. He does show that he is a loyal friend and a supportive teammate. Yamaguchi can at times be seen defending Tsukishima from things he believes were insults toward his friend.

Originally, Yamaguchi constantly doubted his abilities and had difficulty handling his nerves. It was through hard work and seeking mentorship through Shimada that he was able to better his skills and become more confident. By the time Karasuno had their rematch against Aoba Johsai in the semifinals, Yamaguchi has made exceptional progress in both his gameplay and emotions.