Yamaguchi's a pretty shy person who relies on others early on. Despite this, he's generally a nice person but tends to join Tsukishima in teasing others. He's a loyal friend & a supportive teammate, sometimes seen defending Tsukishima. Initially, Yamaguchi constantly doubted his abilities & had difficulty handling his nerves. 
Through hard work & seeking guidance through Shimada, he's able to improve his skills & become more confident. He has made tremendous progress both in his game & emotions.