Yamaguchi is a first year student at Karasuno High School and a middle blocker. He usually clings to Tsukishima after Tsukishima saved him from bullies at 3rd grade, despite being told to shut up numerous times. He is shy, but friendly, and tries his best to contribute to the team. With the help of Makoto Shimada, he has recently become a pinch server due to his ability to do the "jump float serve".