Standing at 166 cm, Yaku is insecure about his height, and his teammates have once stated that it's a taboo subject after Lev pointed it out (he was subsequently kicked by Yaku). He has light brown hair and eyes, and is often seen wearing his Nekoma jersey or tracksuit.
Yaku is respected for his abilities as a strong libero. Karasuno's libero, Nishinoya, had confronted him about his talent as a libero from his experience of playing against him in the same position. He takes great pride in playing as libero, and considers it a huge honor to play as libero for Nekoma, a team whose members specialize in powerful defense.
Off the court, he is supportive of his team and often takes on a parental role when speaking toward, or of them, similar to Sugawara. Because of this, Yaku and Sugawara get along well.
He can also be incredibly short-tempered, especially concerning Lev, who seems to find Yaku's angry outbursts "scary" (although these outbursts are typically in response to Lev tactlessly commenting on Yaku's height and/or attempting to skip practicing receives with him). Even Kuroo at times refers to him as a "demon senpai". Overall, however, Yaku is a rather honest, straightforward person who's very confident about his skills and looks out for his team. He will frequently engage in light-hearted scuffles with his juniors, and will sometimes eat their food too. Fans have remarked that his personality has many similarities to Sugawara's.
Karasuno High Team Formation ArcEdit
Yaku is introduced as Nekoma High's third year starting libero as he is breaking up an embarrassing encounter between Karasuno's Tanaka and his teammate, Taketora. He later encounters Nishinoya, and is taken aback by the latter's apparent sense of humility and constant drive for self-improvement, a quality that he both admires and finds "terrifying".
Tokyo Expedition ArcEdit
Yaku left a rather huge impact during the Tokyo Expedition arc, during which the role of the libero's position was explained in depth; during the Tokyo representative match between Nekoma and Fukurōdani, Nishinoya explains to Hinata that truly skilled liberos are players that manage to not stand out, but whose presence on the court ensures their teams' victory. Even Bokuto stating that Yaku's receives "give him chills" and he acknowledges Yaku's impressive defensive. This in turn inspires Bokuto to play even harder against Nekoma during the Tokyo Representative deciding match.
After losing to Fukurōdani, Nekoma faces off against Nohebi High School for the third and final Tokyo Representative spot. During this match, both teams demonstrate impeccable persistence in maintaining their defenses, despite Nohebi's penchant for attempting to throw Nekoma off with a constant stream of feints, block-outs, and well-timed taunting. At one point, Yaku admonishes an increasingly frustrated Lev and tells him not to flail his arms aimlessly in an attempt to block the opposing spikers, as this makes receiving the ball extremely difficult; Lev angrily responds in turn that despite that, Yaku would certainly save the ball without fail (this outburst causes Yaku to be quite taken aback).
Later in the first set, Yaku leapt into the crowd in an attempt to save the ball which had been received off-kilter by Taketora--this successful display of athleticism and fast reflexes sent the crowd and his team into a frenzy, and ultimately allows Nekoma to score. However, in the process of saving the ball, Yaku landed on a spectator's foot and sprained his ankle. He attempts to shrug off his teammates' concern but grimaces in pain when he tries to walk it off--he is then promptly led off the court for first-aid treatment, and Shibayama is called into the game as the back-up libero. In response to Yaku's tearful frustration, his fellow third years Kuroo and Kai tell him not to worry, and to consider being benched as a "brief vacation" while his team picks up the slack and secures a spot at the nationals. While sidelined and receiving first aid, Yaku continues to watch his team closely, and swells with pride when Shibayama successfully adapts to his role as libero. He tearfully laughs and hugs his teammates after Nekoma secures their victory against Nohebi.
Yaku's skills as a libero rival those of Nishinoya. He has a keen sense of receiving and can steadily receive Asahi's strong spikes despite having never faced him before, a feat which instantly earns him Nishinoya's respect during Karasuno's first practice match with Nekoma. Nishinoya has commented that one reason he admires Yaku is that he does not stand out much on the court, yet his defensive prowess is undeniably invaluable. Akane Yamamoto had also described him as the "Guardian" and "Commander-in-Chief" of Nekoma's defense, and regards him as a player just as important as the ace. His abilities have been praised by Bokuto, who has stated that "Yakkun's receives give me goosebumps!"
Jumping Reach: 301 cm
They first met during the Nekoma vs Karasuno practice match when Taketora and Tanaka were causing trouble. Sugawara and Yaku apologized to each other for their teammates and soon bonded over their similar positions of being caretakers for their teams.
Yaku and Sugawara have a similar personality type, and are seen conversing in a friendly manner on multiple occasions.
Although Kuroo and Yaku played against each other in middle school, they didn't officially meet until they joined the volleyball club during their first year of high school. Because Yaku's team had wrecked Kuroo's and Kuroo was much taller than Yaku, their relationship started off very badly. They would often bicker and disagree on minor things, such as girl's hairstyles (Yaku prefers girls with short hair), summer activities, favorite foods, and even which brand of volleyball they preferred.
One day during practice, all the freshmen members had to say their goals and aspirations out loud, according to the club's tradition. To everyone's surprise, both Kuroo and Yaku declared in unison that they wanted to "dominate nationals", despite their other teammates settling for making into Tokyo's top eight. This declaration ultimately helps them to put aside their differences and unite as teammates who trust each other. Kuroo has been shown calling Yaku various nicknames such as "Yakkun" and "Demon Senpai". At present, their interactions are far friendlier than when they first met.
- Favorite food: Stir-fried vegetable.
- Current concern: His kouhai are causing him all kinds of worry.
- His birthday falls on World Cat Day.
- His star sign is Leo.
- He is in a college prep class.
- Unlike Kuroo, he prefers meat, girls with short hair, spicy curry, Molten volleyballs, cats and going to the mountains during the summer.
- Yaku is the second oldest of the third years.
- While people usually call him "Yaku", Kuroo and Bokuto were shown calling him "Yakkun", Lev calls him "Yaku san", and Nishinoya calls him "Morisuke" .
- He was chosen as the libero on Furudate's Dream Team.
- In Haikyuu's first popularity poll, Yaku placed 14th with 2,211 votes. In the second, he dropped to 19th with 2,979 votes.
- Morisuke (衛輔) - Protecting Help
- Yaku (夜久) - Long Ago Night
- "Yamamoto, stop immediately picking fights with people. It makes you look like an idiot." (To Taketora, Volume 4 Chapter 27)
- "[About Nishinoya] His skills as a libero are at such a high level, and yet far from being conceited, he just looks intently forward... That's pretty frightening." (To Sugawara, Volume 4 Chapter 34)
- "You know, getting nicks and bruises... that's the same as breathing for me. And this entire year, I've managed to avoid any serious injuries or illnesses... So why of all days did it have to be today?" (To the Nekoma team, Volume 22 Chapter 199)