Yousuke Koiwai is the adoptive father of Yotsuba and one of several male characters in the manga. He is depicted with a laid-back personality and, as such, works as a stay-at-home, freelance translator, although it is uncertain what exactly he translates. During a conversation with Fuka, it was made apparent that Koiwai had discovered Yotsuba while travelling abroad and that he had eventually come to raise her as his own.
Although he may be nonchalant in telling other people that Yotsuba is, in fact, a bit odd, he can be very offbeat himself and has a tendency to behave in a plethora of unabashedly silly ways; Mr. Koiwai normally acts as such in order to lighten up the mood and/or when he is humorously emphasizing a point.
He is depicted as a youthful dad with the carefree lifestyle of a slacker who habitually wears an undershirt coupled with a pair of boxer shorts since he is just at home most of the time; he often feels sorry for himself and is apologetic to other people who catch a rare glimpse of his "irresponsible" getup. Koiwai even apologized at how much more "together" Fuka was then he in the beginning of the manga.
Koiwai also seems to have no problem with Yotsuba exploring by herself and makes her run errands when he is too busy to complete a task himself. Koiwai, in fact, had no clue as to where Yotsuba was while he was first setting up his living accomodation and showed few, if any, concern for the situation, as apparently, it was nothing new to him. Despite his somewhat digressed parenting skills, he still cares much for Yotsuba and does aim to be a good father and proper role model to Yotsuba; he will duly reprimand and chastise his child whenever she causes an unfavorable situation that definitely warrants a firmer disciplinary course of action.
Every once in a while, he does get out of the house if it is required by his job or when doing errands such as grocery shopping; he would also gladly accept friendly invitations for outdoor activities and/or night-outs if his quite hectic schedule could allow it. He usually brings his little girl along in these trips which helps add a unique flavor to each encounters given Yotsuba's infallible eagerness and exuberance.
- Yotsuba: The circumstances of her adoption are obscure—Koiwai tells Fuka that without intending to, he found himself taking care of her while somewhere overseas. It is implied that the two had lived together in his mother's house before moving to the suburbs during the beginning. Koiwai seems to be comfortable with Yotsuba at home, as he will often join in on her playful tendencies. On one occassion, he is seen donning his boxers on his head, dubbing himself "Boxer Man" in front of Yotsuba.
- Jumbo: Apparently, Koiwai had met Jumbo when the two were both children living in the country (along with Yanda). Jumbo frequently visits the Koiwai residence and seems to have a great relationship with Yotsuba. Koiwai and Jumbo usually enjoy passing time by residing in front of the TV or just simply slacking together.