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.