Yamaguchi is a shy boy who depends on others, he cannot do things alone, but when he has decided he uses all his courage to do it, as shown in training, where he made Tsukishima start using all his strength in workouts. Even though he needs other people, Tadashi loves volleyball. After the loss against Aoba Johsai, he gained a lot of confidence in himself thanks to his teammates and Makoto Shimada, who taught him how to use the floating serve.