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Yousuke Koiwai
Kowai with Yotsuba
Koiwai with Yotsuba in a volume cover
Japanese 小岩井 葉介
Romanized Koiwai Yōsuke
Gender Male
English voice actor None
Japanese voice actor None

Yousuke Koiwai (小岩井 葉介 Koiwai Yōsuke), or "Dad" (とうちゃん tō-chan), is Yotsuba's adoptive father. The circumstances of her adoption are obscure — Koiwai tells Fuuka that without intending to, he found himself taking care of Yotsuba while somewhere overseas, but gives no other details.

Yousuke grew up somewhere in the country (as told to Mrs. Ayase), where his mother and father still live and where he and Yotsuba stayed until they moved to their current location. His father seems to be in poor health, since his mother remarks that he needs her to help him with all kinds of daily chores and that he used to be hospitalized. He also has a younger sister, Koharuko, who lives in Tokyo. Jumbo claims that Yousuke has been to a lot of places around the world and presumably it was during one of these travels that he first found Yotsuba.


Skärmavbild 2013-03-10 kl. 23.11

Koiwai as he appears in "Try! Try! Try!".

Skärmavbild 2013-03-10 kl. 22.33

A close-up view of Koiwai's face.

Koiwai first appeared in the one-shot manga and webcomics called "Try! Try! Try!" along with his daughter Yotsuba and the Ayase girls, as the only male member of the cast. His appearance and personality were largely similar to that of the beginning of Yotsuba&!.


Yousuke is a rather young man of medium build with shoulder length dark hair. During most of the series, Yousuke is seen to be wearing a white T-shirt and jeans alongside a pair of normal shoes. At some point in the series, Yousuke is wearing a type of beanie with a scarf, shirt, collared jacket, sweatpants, and his classic set of shoes.

While he is at home, he is seen wearing the same white T-shirt though with a pair of shorts. During this time, he is usually seen to be barefoot. During chapter 7, he is seen goofing off with Yotsuba meanwhile Fuuka watches. He is seen wearing a pair of pants on his head as well as only wearing his shirt and boxers.


Although Yousuke acknowledges Yotsuba is a bit odd, he can be offbeat himself. He is depicted as something of a slacker; he habitually wears an undershirt and boxer shorts when working at home, and apologizes more than once for being "irresponsible". He has stated himself that he is also prone to forgetting things.


Koiwai 4851

From a volume back cover

Yousuke works from home as a translator (honyaku-ka in Japanese), though what languages and materials he translates is unknown. This profession was humorously reinterpreted as "trainspotter" (or konnyaku-ya, meaning konjac maker, in the Japanese version) by Yotsuba. It appears that this work provides sufficient income to support himself and his family, and keep them in a fairly nice home. He is also very disorganized, as his house is seen. Regardless of his skills and lack of experience as a parent, he tries to be a good father to Yotsuba. Yousuke is rather sarcastic in his manner of speech, especially around his daughter, though not in an unfriendly way. He is generally nice and polite with other people, such as his neighbors, but is more wacky and laid back with people he is close with.


Yotsuba Koiwai[]

Koiwai and Yotsuba

Koiwai with Yotsuba, his daughter.

Koiwai deeply cares for his adopted daughter Yotsuba and he is the closest person to her in the series. He is aware of Yotsuba's unpredictable behavior and is often worried about Yotsuba endangering herself or disturbing others, particularly the Ayase family. Because of this, he often finds himself apologizing for her actions. Despite the trouble it causes him, he finds her hyperactive and cheerful behavior endearing and humorous, and often plays along with her. This often results in both having their own adventures and playing ridiculous games, such as becoming a made-up superhero called "Pants Man" by wearing his boxers on his head. While Yousuke is pretty lax as a parent, he is strict about some things to Yotsuba, such as never being allowed to touch his computer, go anywhere on her own unsupervised, and lying to him. However, he acknowledges that it is okay for her to make mistakes as it is "her job" to do so, so long that she does not lie to him about it. Besides playing with her, Yousuke also likes taking his daughter to recreational places such as the farm and the beach among others.


Jumbo is Yousuke's closest friend. Apparently they have known each other since childhood since Jumbo at one point asks Yousuke if he remembers a place to which they took a field trip when they were kids, though he fails to recall the event. Jumbo lives in the same prefecture as Koiwai and Yotsuba and often visits them. They also often participate in activities together such as going to summer festivals and stargazing. Though both of them typically get along well, Yousuke is sometimes annoyed at Jumbo's behaviour such as the way he deals with his crush on Asagi (often using Yotsuba as a way to spend time together with her) or his spontaneous trip to Hawaii that he went on only to "win" over Miura, which Yousuke finds ridiculous.


Both kids

Koiwai's thoughts on Yotsuba and Yanda's childish behavior.

Yanda is a friend of Yousuke and is the only male friend of his shown other than Jumbo. Yanda seems to be acquainted to both Yousuke and Jumbo previously, although how is currently unknown. They seem to have a good relationship since Yousuke is alright with inviting Yanda to different activities involving his family and friends, such as going to the Water World together (though Fuuka and Ena ended up going instead), or visiting the ranch. Despite that, Yousuke is often annoyed when Yanda tries to forcibly join in their activities. He also realizes how childish Yanda can be especially around his daughter Yotsuba, who Yanda loves to tease. Yanda seems to be the lowest in the pecking order amongst the men, since Yousuke and Jumbo make it a habit to tease and bully Yanda whenever the three of them get together with Yotsuba often joining in.


Fuuka is the first of the Ayase family Yousuke encounters and arguably the one he has had the most interaction with throughout the series. His initial impression of her is that she is a very mature, bright and polite person, which makes him feel somewhat guilty for lacking those qualities himself to a certain extent. This would later change as he got to know her, with Yousuke reversing his first impression of her. He becomes more casual with her such as calling her a "bishoujo" to annoy her, commenting on her weight, and engaging in friendly but petty arguments with her about trivial things such as what to put in curry. Yousuke also often takes Fuuka along on his trips with Yotsuba, although on some of these occasions Fuuka invited herself, much to his dismay. He was also humorously mistaken for Fuuka's husband by her classmates.


Yousuke and Asagi have a friendly relationship, though their interaction is quite minimal until Asagi invited him and Yotsuba along to a hot air balloon festival. In contrast to Fuuka's more uptight personality, Asagi's casual and open personality is more similar to his, and the two are quick to become friends. Her friend Torako also comments on this, noting that the two of them get along quite well.

Mrs. Ayase[]

Yousuke is grateful to Mrs. Ayase for always taking care of Yotsuba. He feels guilty however, for Yotsuba frequently imposing on her family, though she assures him she doesn't mind. In return, Yousuke takes the Ayase daughters along on trips.


Yousuke has an easy time getting along with his sister Koharuko, due to their similar temperaments, but his lesser maturity and responsibleness is an occasional point of minor friction between them. When Koharuko reacted negatively to hearing he and Yotsuba had eaten crêpes shortly before the three of them were about to have lunch together and asked if they had eaten cotton candy as well, Yousuke falsely claimed that was not the case, a lie that was later accidentally exposed by Yotsuba. Yousuke sometimes teases his sister about her eccentricities or draws attention to them, such as her perfectionism and her unconventional way of eating at a buffet. Despite being the older sibling, Yousuke is shown to be outwitted or intimidated by his sister, such as when he hid the fact that he had eaten before their appointment or when Koharuko sneakily got Koiwai to pay for an expensive meal they had with Yotsuba at a fancy restaurant that had not been part of their initial agreement.


  • The name Yousuke means "leaf" (葉) (you) and "forerunner, herald" (介) (suke).
  • Yousuke's surname Koiwai means "small" (小) (ko), "rock" (岩) (iwa), and "well, mine shaft, pit" (井) (i).


  • Koiwai is 175 cm tall (5'9'').[1]
  • Koiwai's given name was only revealed in Chapter 87 of the manga, over 12 years after the start of the series.
  • Koiwai's favorite food is curry, and Yotsuba also loves it. They even made up a "curry dance" together to celebrate eating it.
  • Koiwai is an ex-smoker, and now has a negative attitude towards smoking, as shown by the fact that he told Yotsuba "only hardliners and bad people smoke." Yotsuba commented, "Daddy used to be a hardliner," when she talked to Torako about this.