Tadashi Yamaguchi (Japanese:
- 1 Appearance
- 2 Personality
- 3 Background
- 4 Plot
- 4.1 Karasuno High Team Formation Arc
- 4.2 Interhigh Arc
- 4.3 Tokyo Expedition Arc
- 4.4 Spring High Preliminary Arc
- 4.5 Tokyo Nationals Arc
- 4.6 Final Arc
- 5 Statistics
- 6 Relationships
- 7 Quotes
- 8 Trivia
- 9 References
Yamaguchi is a young-looking boy with an average build. He has dark grayish-green hair with a flyaway piece of hair sticking up at the top. 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.
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, he tends to snicker along or join in with Tsukishima whenever he's mocking others, especially Kageyama and Hinata. This also proves that he is a loyal friend who cares a lot for his friends, constantly shouting encouragements from the sidelines and defending Tsukishima.
When he was young he was bullied for his freckles/acne which led to him being extremely insecure. He constantly doubts his abilities and finds it hard to concentrate with a large crowd. He is also easily nervous, as he always seem to jump whenever the referee blows the whistle for his serve.
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 Aoba Johsai 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.
Back in elementary school, Yamaguchi was teased for his freckles and overall scrawniness. Once Tsukishima saved him from some boys who bullied him by calling them pathetic. Despite thinking that he was included among the ones Tsukishima insulted, Yamaguchi still grew attached to him and looked up to him, he set his goal to become "as cool as him". He is later revealed to want to join a sports club but decided against soccer and basketball because the other players were 'big and scary' and volleyball seemed a safer choice.
Karasuno High Team Formation Arc
Yamaguchi and Tsukishima join Karasuno Boys’ Volleyball Club a few days after it begins for the new school year. That evening, they visit Hinata and Kageyama at the field where they’re practicing. Tsukishima mocks the two while Yamaguchi stands off to the side, smirking along. When Hinata asks about Tsukishima’s height, Yamaguchi answers for him, irritating Tsukishima. On the way home after this, Yamaguchi asks Tsukishima if something's wrong and Tsukishima angrily replies that he hates hot-blooded people like the other two first years.
3-on-3 First Years Match
On the day of the match, Tsukishima begins by insulting Kageyama and Hinata and Yamaguchi hurriedly warns Tsukishima, only for the latter to retort that he’s using provocation on purpose. Yamaguchi doesn’t show much of his abilities during this match as Daichi covers the receives and Tsukishima covers the blocks. In the end, his team loses to Kageyama and Hinata's
At the end of the match, Yamaguchi calls out to Tsukishima worriedly as Hinata attacks him in an attempt for a handshake. Yamaguchi gets his team jacket and hesitantly poses with Tanaka and the other upperclassmen.
Karasuno Neighbourhood Association Match
Yamaguchi didn't participate but at the end of the match he approached Makoto Shimada and asked him to teach him how to do a jump float serve.
Aoba Johsai Practice Match
He is seen with Kageyama, Tsukishima, and Tanaka intimidating of Shigeru Yahaba and another member of Aoba Johsai. He does not play in the match. He is upset by the fact that he is the only first-year not playing.
Karasuno vs Aoba Johsai
Before the match is about to begin, Karasuno warms up. Yamaguchi is practicing his serve and Takinoue, who is watching, notices Yamaguchi and asks Shimada next to him how he’s doing as his apprentice. Shimada reminds Takinoue that they’ve only been practicing together for a week, but also tells him that Yamaguchi would manage a successful serve as a fluke from time to time, but acknowledges that it’ll take some time before he can make them with consistency. During the match, Yamaguchi notices that Kageyama is acting differently from his habitual self. After Sugawara is subbed in for Kageyama, Hinata is delighted that he gets to stay on the court longer than Kageyama, and at this, Yamaguchi holds his teammate back, afraid that Hinata gets them a warning.
During the third set, Yamaguchi notices that Hinata is eager to go out and play, and he tries to stop him from getting Karasuno a warning. Despite this, internally, Yamaguchi admits that he also wants to get out of the warm-up zone and play. Yamaguchi is still eager to play even when seeing his exhausted teammates. However, when Yamaguchi is subbed in as a pinch server, he is extremely nervous. He relaxes when seeing his teammates cheering for him but tenses when looking at Aoba Johsai on the other side of the court. As he serves, he surprises Iwaizumi and Oikawa with a jump floater but fails the serve and he holds back tears as he subbed out.
Later, he talks with Makoto Shimada, his unofficial volleyball guide, about the match, who advises him about his choices in volleyball and reveals to him about his own experiences with volleyball. This motivates Yamaguchi to try harder in volleyball next time.
Tokyo Expedition Arc
Throughout the Arc, he is seen multiple times practicing his float serve, mostly with his teacher Makoto Shimada, or Nishinoya.
When Kageyama and Hinata have trouble studying he is the one to convince Tsukishima to help them and is later seen helping them as well. When Yachi is first introduced to the Karasuno players, Yamaguchi thinks of her as "cute". He then recommends Hinata to ask her for help studying since she is in an advanced class.
During the training camps in Tokyo, he would mostly focus on trying to improve his serve and later asks Tsukishima to help him practice which he declines. He then asks if he is going to practice anything by himself to which Tsukishima declines as well. Yamaguchi, worried about Tsukishima's lack of motivation, asks Hinata what he would tell Tsukishima. Hinata answers that he wouldn't say anything since he can`t even tell if Tsukishima likes playing volleyball in the first place. Yamaguchi explains that he thinks Tsukishima doesn't hate volleyball, since he wouldn't have come to Karasuno otherwise.
In the next episode, we are shown a series of Yamaguchi's flashbacks. In the first flashback, we see him being bullied by three kids. Tsukishima passes by and upon noticing exclaims the phrase "lame". Although Yamaguchi isn't sure what exactly Tsukishima was referring to he still admires him for being able to say it. Later, he meets Tsukishima at the Volleyball Gym. He tells him that he came to check out volleyball since it seems safer than other sports. Tsukishima, in return, tells him about his brother. In the last flashback, Yamaguchi is present when Tsukishima finds out his brother had been lying to him about being the Ace of the Karasuno team.
After Tsukishima decides to blow off extra practice once more Yamaguchi confronts him. He tells him that he has always admired his height, intelligence and coolness, but that his lack of trying to improve makes him "lame". When Tsukishima asks him how one can still give his best even though the chances of ever becoming the very best are almost non-existent Yamaguchi, remembering his failure against Seijoh, angrily exclaims "What else do you need besides pride?". Tsuki, obviously impressed then calls him "actually cool" and heads off to talk with Bokuto. After this Yamaguchi asks Hinata if he sees Tsukishima as a rival, to which he answers "obviously". When Tsukishima gives is his all and forces Bokuto to use a faint in the match against Fukurōdani the next day Yamaguchi is seen on the sidelines smiling.
Spring High Preliminary Arc
Karasuno vs Kakugawa
Yamaguchi did not play in this match but is shown to have narrowly avoided being hit by a powerful spike from Hyakuzawa before the match started.
After the preliminaries, Yamaguchi is seen continuing to work on his serve and finally managing to perform a float serve that Nishinoya is unable receive.
Karasuno vs Johzenji
Yamaguchi mostly supports the team during the match as he does not participate in any plays. When Kageyama received a nose bleed from having gotten hit with the ball, Yamaguchi escorts the setter to the medical station and quickly returns to the game.
Karasuno vs Wakutani South
Yamaguchi gets switched in. He is once again very nervous even though Tsukishima and Ennoshita try to cheer him up. Despite their efforts, Yamaguchi is still on edge with is evident when he flinches at the sound of the whistle. This results in his serve toss being too low and the ball barely getting over the net. Although the serve still scores a point and his first service ace, Yamaguchi becomes too worried and hits his next serve without jumping in order to get it over safely. Karasuno still scores the point and wins the set but Yamaguchi is visibly upset about backing down. Ukai, who is returning from helping an injured Daichi, angrily walks up to him, ready to give him a lecture, but Ennoshita steps in. He explains that Yamaguchi already knows better than anyone else of what he's done.
In the stands, Shimada explains to Saeko that a jump serve offers much more speed and efficiency and that Yamaguchi's only option to fight on equal grounds with his teammates is to give it his all with his best and only weapon. After the match, Yamaguchi leaves the rest of the team for unknown reasons.
Karasuno vs Aoba Johsai
Just before the match, a nervous Yamaguchi tells Yachi about his failure during the last match against Seijoh, which makes Yachi nervous. Though frightened that he unintentionally made Yachi nervous, Yamaguchi was still able to overcome some of his nerves.
During a 23-19 lead by Seijoh in the second set, Yamaguchi silently urges Ukai to switch him in. A flashback is shown of Yamaguchi seeking out Ukai to apologize for backing down during his serve in the previous game, even though Ukai had already expressed his opinion on this during the Fukurōdani training match (Asahi did not do a jump serve in order to play it safe). He asks him to give him another chance.
Back to the Seijoh match, Yamaguchi is switched in and gets told by Hinata to score ten points. On his note that that would end the match, Hinata tells him that he would forgive him. As he gets ready to serve another flashback show Shimada reminding him that he should wait some of the allowed eight seconds after the whistle to calm him. The referee gives the signal and Yamaguchi is able to stay calm and deliver his serve.
The ball looks to be out but curves downwards, turning into a service ace. The team celebrates him and Tsukishima notes that his serve wasn't surprising, given all the work and practice he put in over the last several months. Yamaguchi's second serve curves once again, hitting Kyōtani in the arm and scoring another service ace. Although the third serve is returned via an overhand hit, Yamaguchi receives Iwaizumi's spike with his shoulder and Tsukishima scores the point. Tsukishima hands Yamaguchi the ball back with a faint smile. After a time-out from Seijoh, hits his fourth serve. This time, Yamaguchi decides for a more offensive approach by aiming for the top of the net, which gets him his third service ace. His fifth serve has a loose spin and doesn't change direction, but Asahi still scores a point off the returning ball. Even though his sixth serve turns out good, Seijoh returns it and scores, finally stopping Yamaguchi's streak. Off court, he is complimented by the coach and his teammates and tells Hinata that he will score ten points next time.
After a strong serve from Oikawa, Yamaguchi compares his serves to that of the Seijoh captain, admiring his power and accuracy even during a tense situation.
In the final set, he gets switched in once more; hitting another curving float serve, but Seijoh barely manages to pick it up and score. Oikawa is internally amazed by Yamaguchi's progress and Ukai compliments him once more, saying that it was a great serve that could only be stopped because of Seijoh's very strong play.
Karasuno vs Shiratorizawa
During the middle of the first set, Yamaguchi gets alarmed when he sees that Tsukishima’s finger was slightly injured and when expressing his concern, he makes his sentences rhyme. Tsukishima tells him that this bothers him and Yamaguchi apologizes. As the set is wrapping up, Yamaguchi is subbed in and manages to score. During the second set, Karasuno is informed of Tendo’s read blocks. Yamaguchi wonders how Kageyama could do to keep his setting being kept from being read, and sarcastically suggests that he does some kind of crazy dance. Shortly after, Yamaguchi would try sending “subtle” signs to Ukai to indicate to him that he wants to play. When he is finally subbed in, Yamaguchi tells himself that he doesn’t have the confidence to tell Tsukishima Hinata’s line to Kageyama about being the greatest when he’s around but he knows that at least he’s come far enough that he can help Tsukishima. When the rally goes to Shiratorizawa, Yamaguchi leaves the court, upset.
After Tsukishima wins Karasuno their first set, Yamaguchi is happy. He then tells Sugawara that he expected Tsukishima to do something like that because of his competitive nature. Near the end of the third set, Yamaguchi is subbed in but his serve goes out. Near the end of the third, Sugawara, Yamaguchi, and Narita talk about how Ushijima confronted Hinata. Yamaguchi and Narita think that Ushijima was being too hard and that Hinata was just playing the victim, but Sugawara retaliates by telling them that anyone who would’ve been hurt by such comments would never have gotten as far as Hinata did. When Tsukishima is subbed in for Hinata, Yamaguchi tries to motivate his friend by yelling not to let Hinata surpass him, but this only annoys Tsukishima.
Yamaguchi would then be subbed in as pinch server. Yamaguchi aims at Ushijima, but his serve instead goes to Reon. However, Ukai already had a retaliation to counter this, and this was to at least make Shiratorizawa badly pass the ball, and in turn, Ushijima’s passes would be rough as well. However, when Yamaguchi serves again, Ushijima smashes the ball right past him. During the fifth set, Yamaguchi is subbed in again. He plans to aim at Ushijima, but the ball touches the net. At his third serve, Yamaguchi’s serve is too long. However, Goshiki does not notice this and tries to receive the ball but it goes out. Following Karasuno’s victory, Yamaguchi tries to make Tsukishima feel better about being upset over only having blocked Ushijima once.
Tokyo Nationals Arc
When the team is having their measurements done, Yamaguchi reveals to Hinata that he has grown and is currently 180 cm. He would go on to congratulate Tsukishima for finally making it just over the 190 cm mark but would be startled when Tanaka tackled Tsukishima in annoyance of the first year's height. As the team is measure their spiking heights, Yamaguchi assists Yachi when she is determining the measurements.
When Takeda announces that Kageyama was invited to the All-Japan Youth Camp, Yamaguchi expresses great surprise at the news and would show joy when Tsukishima was invited to the First Year training camp at Shiratorizawa Academy. After one night of practice, Yamaguchi tells Tsukishima how he would like to attend the youth camp and Tsukishima jokingly suggests that he should crash the camp; the two being unaware that Hinata was listening to their conversation. On the first day of the respective camps, Ennoshita informs the team that Hinata had been caught at Shiratorizawa which left Yamaguchi and many others speechless at his actions.
During the week, Yamaguchi is mostly seen working on his jump float serve and begins to form a sort of one-sided rivalry with Kinoshita when it was becoming apparent that Kinoshita's jump float serves were starting to rival his own and even at times getting by Nishinoya. After Kageyama, Hinata, and Tsukishima return, Karasuno has a practice match against Date Tech. During that time, Yamaguchi would be overwhelmed at seeing Date Tech's newly improved blocking. While his eagerness to get into the match was noticed by Ukai, the first year would not take part in the match as a pinch server.
Eventually, the team heads to Tokyo for the Spring Tournament. When Ukai presents the team with a video made by Takinoue, Yamaguchi would tell Yachi that it made him feel more relaxed when she asked why he was not joining the other first years during a run. The next day, the team would have to leave their warm-up early due to the game before them ending sooner than expected. During a brief period when Hinata made a run to the bathroom, Yamaguchi volunteered to watch his belongings but was momentarily distracted and didn't notice a young boy accidentally taking Hinata's bag containing his sneakers. When the discovery is later made, Yamaguchi blamed himself for the mishap but expressed great relief and gratitude to Kiyoko for retrieving the shoes and making it back in time for their first game.
Karasuno vs Tsubakihara
In the game against Tsubakihara, Yamaguchi is sent in early to try to help close the point gap. Realizing that he will most likely just be serving during the tournament, Yamaguchi remembers what Shimada taught him of using something around him as a reset point to calm his nerves. Using an exit sign across the stadium, Yamaguchi's nerves settle and he aim's his serve to go behind Teradomari in the hopes of containing the ace. Although the serve is received by another player, Teradomari is forced to make the second touch and was therefore kept from attacking. When the Karasuno blockers shut down the attack, Yamaguchi is up to serve again and scores a service ace when Maiko fails to receive. Tsubakihara manages to score in the next rally and therefore cut off Yamaguchi's serve.
Karasuno vs Inarizaki
Yamaguchi is subbed in as a pinch server when Hinata is up to serve during the game against Inarizaki. He mentally goes through the plan of trying to aim his serve directly at Aran in hopes of getting him to lunge or dive or anything to get the ace down to one knee. Remembering how Shimada gave him the tip of using something to help reset his serves, Yamaguchi looks for the exit sign that he had used during the match against Tsubakihara only to find that the sign is being blocked by banners from another school. He starts to panic but Shimada appears below where the exit sign is and holds up a bag from his market to help Yamaguchi reset his emotions. Thankfully Yamaguchi is able to do so and gets a service ace when Aran is unable to receive. His next serve is picked up and countered, thus ending his serve.
Yamaguchi is later sent in for Hinata once again during the second set after Kita's serve is stopped. However, Kita is the one who receives Yamaguchi's serve and the counter attack leads to Yamaguchi being stopped once more. In the third set, Yamaguchi is again swapped in when Hinata moves to the back row. Aran receives followed by Osamu and finally Suna getting the last one over. Inarizaki eventually ends Yamaguchi's serve and he is taken off the court.
When Karasuno wins the match, Yamaguchi is seen rushing toward Tsukishima and in the sub-arena points out that Tsukishima is attempting to put on a second jacket from how weary he is.
Karasuno vs Nekoma
While the match was still in the early stages, Yamaguchi is sent in. At noticing that Kageyama hadn't been able to get a service ace off of Nekoma, Yamaguchi hopes that he will be able to be the one to accomplish this. He is able to do so when Yamamoto is unable to receive his float serve. The serve not only put several of the Nekoma players on edge, it also annoyed Kageyama at not getting the first service ace for Karasuno. His second serve would be received but would throw Nekoma off just enough for Tsukishima to figure out who would be the next attacker. Tsukishima is able to block Fukunaga in a text book example of a serve and block technique. Yamaguchi and Tsukishima high five in celebration of their attack working. Yamaguchi would be subbed out when Nekoma countered his third serve and he would receive praise from Daichi when Karasuno called a time-out. During the time-out, Yamaguchi recalls how his serves felt and notices that he felt calm during the serves.
When he goes in during the second set, he admits that he never believed someone as timid and easily frightened as himself would ever be standing on the court at a national level. His serve is immediately countered. He mostly acts in a supporting role throughout the rest of the set and is highly impressed that Hinata was able to use his improved jumping to get past a triple block. Yamaguchi is sent into the game against during the final set. His first serve is countered by Tsukishima was able to get an attack by Kuroo. His second serve is stopped and he is removed from the game.
Karasuno vs Kamomedai
As Karasuno’s match against Kamomedai is about to start, Yamaguchi listens as Ukai goes over the players of Kamomedai; specifically Hoshiumi. However, after Daichi tries to motivate the team and says that they have now made it into the top eight schools in the nation, Yamaguchi is intrigued. The team captain then explains that if Karasuno beats Kamomedai and makes it semi-finals, they’d get to play on the center court, which excites the team. Soon after, Yamaguchi and Tsukishima are sitting together, preparing for the match, and Yamaguchi is shocked over how Karasuno made it so far but then tries to motiving Tsukishima but trips over his tongue from talking too fast. Immediately after, the two friends break into laughter.
During the game, Hirugami is quick to score a blocking point, which is something that Yamaguchi is frustrated about but not surprised as they are facing “Hirugami the Immovable”. However, Yamaguchi doesn’t want to dismiss Tsukishima’s blocking either and tells him that he isn’t counted out. Despite this, Tsukishima says that any team is at least going to have at least one standout blocker. Into the match, Tsukishima can block Hoshiumi, which results in Sugawara, Hinata, and Yamaguchi shaking back in forth in surprise and excitement. Not long after, Tsukishima and Yamaguchi notice that Hoshiumi hit it in the opposite direction which he was facing when he served earlier.
When Yamaguchi is rotated in the court to act as Karasuno’s pinch server, his serve goes out of the court and gains Kamomedai a two-point advantage.  Later on, Hakuba wins Kamomedai the first set despite having fallen for Hinata’s decoy trick, Kinoshita expresses his intimidation over their opponent. However, upon hearing this, Ukai tells them not to create vague reasons to scare themselves. However, he is shocked when he finds out that it wasn’t why his team was afraid. Tsukishima mocks Hinata to himself that he had been too slow to figure it out and Yamaguchi had to tell him that they still lost the point. Nearing the end of the match, Yamaguchi is subbed in as a pinch server, and in the end, Karasuno gets the point.
Into the third set, Hinata is revealed to have a high fever and is made to leave the court. Yamaguchi accompanies him on part of the way as he does. After this, the game resumes but Tsukishima gets a leg cramp. Not knowing who to sub him in for, Sugawara stubbornly tells his coach not to worry about not picking him because he’s a third-year and in the end chooses Yamaguchi. After their loss, Yamaguchi shakes Tokura’s hand. 
In a brief coverage of events that took place after the Spring Tournament, Yamaguchi is revealed to have become the team captain during his third year. Yamaguchi attends the match between the Adlers and the Jackals, noting to Osamu that this is the first time since middle school that Hinata and Kageyama will face each other on opposing teams. At the time, he is a college senior and will be working for a home electronics company once he graduates.
Yamaguchi is soon joined by Yachi and the two go to the stands where Tsukishima is waiting. When Hinata scores the first point of the game, Yamaguchi joins many of his former teammates in welcoming Hinata back. He later listens to Tsukishima's theory that the new quick attack that Hinata and Atsumu perform is even faster than the original quick attack. After Hinata is called on to make a set and gives the ball to Atsumu, Yamaguchi and Yachi see that Hinata not only has the skills to be a wing spiker but has also learned how to play as a setter and libero which is why he was able to replace Oriver Barnes who is normally the Jackals canon. This also changes the dynamics of the Jackals plays by having Hinata in the starting line-up. Not long after, Yachi and Yamaguchi both take notice that Kageyama hasn't been using many quick attacks. Yamaguchi believes that this is due to Kageyama having both Nicollas Romero and Ushijima to set to as both players are capable of using pure power to score points.
The Jackals would go on to secure their victory in the fourth set. Although Tsukishima tries to tell them otherwise, Yamaguchi and Yachi promise to attend his next match to support him. Yamaguchi's final appearance in the series is when he is shown to be watching the match between Argentina and Japan while wearing an Olympic jersey that matches Hinata's.
|High School Statistics|
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 Aoba Johsai 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 the 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. 
- Kei Tsukishima: They have known each other since elementary school when Tsukishima saved Yamaguchi from some boys who bullied him by calling them pathetic. Despite thinking that he was included among the ones Tsukishima insulted, Yamaguchi still grew attached to him and set his goal to become "as cool as him".
Yamaguchi calls Tsukishima by a nickname, "Tsukki", though the latter doesn't seem to mind. Although Yamaguchi is generally a shy person, he follows Tsukishima whenever he's mocking other people and snickers along. Yamaguchi also doesn't seem to mind whenever Tsukishima snaps at him, and he even apologizes for his behavior. He seems to care for and admire Tsukishima a lot. This is shown during the match against Aoba Johsai; every time a serve was directed to Tsukishima and he didn't receive it properly, Yamaguchi was freaking out on the sidelines, but after Tsukishima finally received one, he was shown crying in admiration. He also often brags about Tsukishima to other people, though the aforementioned stays silent or tells Yamaguchi to shut up. They are barely seen apart from each other and are even in the same class in school.
At first, it seemed like Yamaguchi was simply following after Tsukishima without being thoroughly acknowledged by him, but Tsukishima truly cared for him and viewed him as a friend. When Yamaguchi succeeded with his jump float serve for the first time ever, Tsukishima complimented him and pointed out his hard work to the other teammates. However, with the time they spent at Karasuno, they grew apart in terms of effort they put into their training: while Tsukishima did only what he needed to, Yamaguchi trained even after team practices to work on his float serve. The situation escalated in the summer training camp as Tsukishima still refused to put in any more effort because he didn't see a reason to. Yamaguchi, who was frustrated by Tsukishima's way of thinking, yelled at him for the first time ever to have pride in what he does instead of giving up like this. This changed Tsukishima, who even called Yamaguchi "cool" for the first time ever, causing their relationship to recover.
- Shōyō Hinata: Though Yamaguchi usually assists Tsukishima in mocking Hinata, Yamaguchi and Hinata develop a friendly relationship as the year goes along. Yamaguchi is always there to try to keep Hinata from getting into too much trouble. When Yamaguchi was at a loss of what to do with Tsukishima during the summer camp arc, he asked Hinata for advice.
During the Tokyo Nationals Arc, he and Hinata seem to have a better friendship. Yamaguchi even followed Hinata to gift shops and to watch the Fukurōdani vs Eiwa match. He was also present when Hinata ran into Kōrai Hoshiumi for the first time.
- Tobio Kageyama: Though originally hated by Kageyama because he aided Tsukishima in picking on the setter, Yamaguchi and Kageyama eventually developed mutual respect for each other. Whenever Yamaguchi was switched into games and succeeded with his jump float serves, Kageyama would compliment him.
- Hitoka Yachi: Yachi and Yamaguchi are good friends who get along well and can relate to each other. When Yachi first arrived with Shimizu to practice, Yamaguchi thinks of her as "cute." He's shy around her at first, but eventually they start to get closer. In the novels, Yachi comments that Yamaguchi is taller than she thought. Before their rematch against Aoba Johsai, Yachi offers helpful advice to Yamaguchi on how to handle his anxiety but ended up becoming anxious herself. When Yamaguchi succeeded with his jump float serve, Yachi seems to be brimming with pride.
Karasuno Neighbourhood Association
- Makoto Shimada: Shimada is the one that taught Yamaguchi the jump float serve which became his forte. After watching Shimada play in the practice match against the Neighborhood Association, Yamaguchi went to his store and asked him to become his teacher. Since then, Yamaguchi often goes to Shimada Mart after closing hours to practice. Shimada acts like an older brother to Yamaguchi as well, often encouraging and cheering him on, and he also calls the younger boy by his first name. When Yamaguchi messed up during the Aoba Johsai vs Karasuno match in the Interhigh, Shimada comforted him and motivated him to get back on his feet.
- "This line [of the volleyball court]... everything beyond this line is a different world." (To himself, Volume 8 Chapter 64 / Season 1 Episode 23)
- " It feels like the temperature on this side [of the volleyball court]... is different." (To himself, Volume 8 Chapter 64 / Season 1 Episode 23)
- "Show them [|Aoba Johsai that you can fight, too!" (As he was about to attempt a jump floater serve during the Inter-high tournament match with Aoba Johsai High, Volume 8 Chapter 64 / Season 1 Episode 23)
- "Up until junior high, I thought it was fun just to play with everyone. That was what volleyball was to me. But that's not it... that's not it... I want to be like the others, and... I want to be able to make my body move right! I want to control the ball! I want to play on the same level as the other, better players!" (To Makoto Shimada, Volume 8 Chapter 70 / Season 1 Episode 25)
- "Motivation? What more do you need than pride?" (To Kei Tsukishima, Volume 10 Chapter 88 / Season 2 Episode 8)
- "What can Kageyama do to keep his setting from being read like that? Should he do what Hinata does and do some kind of crazy antsy dance first to throw them off?" (To Tsukishima, about Kageyama, Volume 18 Chapter 156)
- "I only have one weapon: my serve. I'm going to fight putting my pride and the flow of the game into that shot. I am ... a pinch server." (To himself, during his 3rd service Ace against Aoba Johsai, Season 2 Episode 22)
- "Sorry, Tsukki!" (Various, towards Tsukishima)
- 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