Koiwai is Yotsuba's adoptive father. The circumstances of her adoption are obscure; he tells Fuuka that without intending to, he found himself taking care of her while somewhere overseas, but gives no other details. Though he 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 during the summer, and apologizes more than once for being irresponsible. He works from home as a translator, though what languages and materials he translates is unknown. Regardless of his skills as a parent, Koiwai tries to be a good father to Yotsuba.