Koiwai with Yotsuba in a volume cover
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Koiwai 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. Koiwai's father seems to be in poor health, since his wife remarks that he needs her to help him with all kinds of daily chores, and he used to be hospitalized. He also has a younger sister, Koharuko, who lives in Tokyo. Jumbo claims that Koiwai has been to a lot of places around the world, and presumably it was during one of these travels that he first found Yotsuba.
DevelopmentKoiwai 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&!.
Appearance and personalityThough Koiwai 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. He is rather young, has shoulder length dark hair and is of medium build. He works from home as a translator, though what languages and materials he translates is unknown. He is also very disorganized, as his house is seen. Regardless of his skills as a parent, Koiwai tries to be a good father to Yotsuba. He 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.
YotsubaKoiwai deeply cares for his adopted daughter Yotsuba, and 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 Koiwai wearing his boxers on his head and becoming "Pants Man". While Koiwai is pretty lax as a parent, he is strict about some things, such as Yotsuba never being allowed to touch his computer, lying, and Yotsuba going off on her own without supervision. 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. Koiwai also likes taking his daughter to recreational places, such as the farm and the beach among others.
Jumbo is Koiwai's closest friend. Apparently they have known each other since childhood since Jumbo at one point asks Koiwai if he remembers a place to which they took a field trip when they were kids, though Koiwai 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 Jumbo and Koiwai typically get along well, Koiwai is sometimes annoyed at Jumbo because of such things 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 Koiwai finds ridiculous.
YandaYanda is another friend of Koiwai's, but Yotsuba doesn't think as much of as her father does. Koiwai seems positive about 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, but he also realizes how childish Yanda can be. Koiwai thinks "they are both kids" during an argument between Yanda and Yotsuba. Both Koiwai and Jumbo make it a habit to tease and bully Yanda whenever the three of them get together.
Fuuka is arguably the Ayase Koiwai 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 Koiwai even thinking she wasn't the diligent type, despite having the opposite impression when they first met. After settling down in his new house, he gradually develops a more casual relationship with her, and feels comfortable enough teasing her, such as calling her a "bishoujo" or commenting about her weight, and engaging in friendly but petty arguments with her about trivial things, such as what to put in curry. Koiwai 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.
Koiwai 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.
Koiwai 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, Koiwai also takes the Ayase daughters along on trips.
Koiwai 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, Koiwai falsely claimed that was not the case, a lie that was later accidentally exposed by Yotsuba. This shows that Koiwai is worried about getting scolded by Koharuko in spite of being the older sibling. Koiwai 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. Koharuko sneakily got Koiwai to pay for an expensive meal they had with Yotsuba at a fancy restaurant, which had not been part of their agreement, though Koiwai did not contest the decision.
- Koiwai is 175 cm tall (5'9'').
- His given name was only revealed in Chapter 87 of the manga, over 12 years after the start of the series.
- His favourite 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.