Yamaguchi is a young-looking boy with an average build. He has short black hair, often portrayed as brown or even green with a flyaway piece of hair sticking up at the top. Back in middle school, he was teased for his freckles/acne and overall scrawniness. He seems to have grown into it now, and isn't bothered about it as much.
After joining the Karasuno volleyball team during games, Yamaguchi wears Karasuno's black, orange, and white uniform with the number 12 on the back. He also wears knee pads, and white volleyball shoes with black accents.
Outside of matches and practice, Yamaguchi wears a black gakuran, the Karasuno uniform.
Yamaguchi is a pretty shy person who relies on others. Though he is generally a nice person in the anime, he tends to snicker along or join in with Tsukishima whenever he's mocking others, especially Kageyama and Hinata, in the manga.
He is hardworking, as shown when he sought for extra training on his own and how he continued practicing outside of the team, after seeing how weak he was compared to his other teammates.
After a fatal loss in the match against Aobajohsai High, he gained a lot of confidence thanks to his teammates and Makoto Shimada, who has helped him practice the jump float serve. During the summer training camp, he even stood up against Kei Tsukishima, resulting in him being surprised at Yamaguchi's actions and even calling him "cool" for the first time. Tsukishima started working harder, and their relationship returned to normal after that event.
Yamaguchi grows into a reliable pinch server with his jump float serve. Early on, Yamaguchi began practicing the jump float serve technique, not wanting to be left behind by the other first years. Although attempting and failing the serve against Aobajōsai at a tight moment of the match, he works extremely hard to improve it. He is originally hindered by his lack of experience and weak-heartedness but, since then, has grown much more assertive and confident about his role.
Despite his skills as a server improved over time, his body control as a receiver is still lacking, resorting to body saves and unorthodox shots.
Height and reach, as of mid-November:
- Fingertip Height: 232 cm
- Jumping Reach: 315 cm (spike) / 300 cm (block)
- Jump Float Serve: This serve is Yamaguchi's main weapon and has helped out Karasuno in many clutch moments. His jump float serves can vary from skimming the top of the net to catching opponent off guard having to receive from further out before. Under the tutelage of Makoto Shimada, Yamaguchi increases the speed of his serve so that opponents have a harder time reading the trajectory. He also has worked on improving his aim and accuracy to target a specific player.
- Serve and Block Combo: This strategy involves Yamaguchi breaking the opponents' formation with his serves, thereby forcing players out from participating in offense. This would reduce the amount of information Tsukishima needs to process in order to put up an effective block. The two successfully use the serve and block technique in the match against Nekoma. 
- Favorite Food: Soft, floppy french fries
- Current Concern: Whenever he thinks a cute girl has come over to talk to him, all they want to talk about is Tsukishima.
- His star sign is Scorpio.
- There has been debate over whether the dotting on his face is acne or freckles, but many believe that the specks are freckles.
- In Haikyuu!! Radio #22, Yamaguchi's voice actor, Soma Saito, described Yamaguchi with the word "freckles" in a guessing game with Uchiyama Kouki, Ayumu Murase, and Kaito Ishikawa.
- Since he's in Class 4 with Tsukishima, he must be smart; Classes 4 and 5 are college-preparatory classes in Karasuno. Although he himself has stated that he is terrible at English.
- His original reason for joining a sports club was so he could grow stronger. He chose volleyball specifically because the other sports clubs had "scary-looking guys".
- He is the only first year to not be a regular of the volleyball team.
- He is the second youngest of the first years.
- In the fourth instance of Haikyuu!! Radio, Yamaguchi's voice actor for the anime adaption, Saitō Sōma, was asked "What sort of person is your character, Yamaguchi?" to which he responded "Loves Tsukki very much."
- His seiyuu, Saitō Sōma, is also the seiyuu for Twelve, a character in the anime Zankyou no Terror. Number twelve is Yamaguchi's shirt number.
- In the english dubbing of the anime, he is the only first year, as well as one of few characters, refered to by his first name instead of his last name.
- He shares a birthday with Kenji Futakuchi.
- When asked about Yamaguchi's name origin, Furudate said, “He’s 忠 (loyalty), like a loyal dog!”
- 忠 is also how "Hachiko" is written. Hachiko is a famous loyal Japanese dog who also shares the same birthday as Yamaguchi.
- In Haikyuu's first popularity poll, Yamaguchi placed 15th with 1,993 votes. In the second, he rose to 7th with 6,561 votes.
- Tadashi (忠) - Loyalty; Devotion
- Yamaguchi (山口) - Mountain Entrance