Tadashi Yamaguchi

Height: 179.5→180.0cm / Weight: 63.0kg / Highest jump reach: 315cm / Birthday: November 10 / Favorite food: Soggy French fries / Recent concern: Thought a cute girl was talking to me, but she just kept asking about Tsukki

First-year MB at Karasuno High School. Jersey number changed from 12 to 1 in his third year.

Although an MB, he is used as a pinch server during matches. Skilled in jump float serves.

Childhood friends with Tsukishima, whom he calls "Tsukki," and they usually hang out together (in the same class). Initially, he had a provocative attitude towards Hinata and Kageyama, following Tsukishima's lead, but he is essentially a "good kid" and even teaches Kageyama in studies. He is Tsukishima's greatest supporter and was the catalyst for his change during the summer training camp.

Seeing other first-years participate and excel in matches motivated him to visit Shimada (a Karasuno alumnus) to learn the jump float serve. He made his official match debut as a pinch server during the Interhigh prefectural qualifiers against Aoba Johsai, but failed due to extreme nervousness. However, he never forgot that frustration and continued to practice diligently. In the Spring High preliminaries quarterfinal against Wakutani South, he played it safe due to fear of failure, which led to self-loathing. After the match, he asked Ukai for "another chance," and in the semifinal against Aoba Johsai, he scored three service aces as part of a five-point run, which surprised everyone. Tsukishima remarked that Yamaguchi had practiced serves more than anyone else over those five months, implying his performance was expected. During the serve training week before Spring High, he aimed to learn faster serves and repeatedly scored service aces in the first match of Spring High against Tsubakihara Academy, impressing Ukai with his rapid growth. He served as the captain in his third year of high school. When Hinata returned from Brazil, he went to meet him.

After graduating high school, he attended the same university as Tsukishima and is planning to work at an electronics manufacturer after graduating from university.