山口忠
• Height : - 179.5 cm (April)
- 180.0 cm (November)
• Weight : 63.0 kg
• Birthday : November 10
• Age : 16
• His star sign is Scorpio.
• His favorite food is soft, floppy french fries.
• He plays the role of pinch server in the team.
• 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.
• His current concern is whenever he thinks a cute girl has come over to talk to him, all they want to talk about is Tsukishima.
• Since he's in Class 4 with Tsukishima, he must be smart; Classes 4 and 5 are college-preparatory classes in Karasuno.
• 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".
• Yamaguchi calls Tsukishima by a nickname 'Tsukki' and the latter doesn't seem to mind.
• He is the only first year to not be a regular in the first series of the anime.
• 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.
• Shimada is the one that taught Yamaguchi the jump float serve which became his forte.
• 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.
• Yamaguchi shares the same birthday with Kenji Futakuchi.
• Back in middle school, he was teased for his freckles and overall scrawniness. He seems to have grown into it now, and isn't bothered about it as much.
• In the 4th instance of Haikyuu! Radio, Yamaguchi's seiyuu for the anime adaption, Saitō Sōma was aaked "What sort of person is you character, Yamaguchi" to which he responded "Loves Tsukki very much."
• Yamaguchi and Tsukishima have known each other since elementary school when Tsukishima saved Yamaguchi from some boys who bullied him by calling them pathetic.
Yamaguchi became attached to him and set his goal to become 'cool' like him.
• Although Yamaguchi is generally a shy person, he follows Tsukishima whenever he's mocking other people and snickers along.
• When Yachi first arrived with Shimizu to practice, Yamaguchi thinks of her as "cute."
• Yamaguchi seems to have a small crush on Yachi, as he's seen blushing around her at times.