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Yotsuba's adoptive father.
While the exact circumstances of his adopting Yotsuba are unknown, Mr. Koiwai (first name unknown) did give Fuuka a vague description of how Yotsuba came to live with him, saying that he met her in a foreign country and found himself raising her before he knew it.
Koiwai usually works at home as a translator, and has an easy-going, slacker-ish demeanor. He is prone to walking around his house in his boxer shorts, takes a lackadaisical approach to housework and has a bit of a laissez faire attitude to Yotsuba's activities. Despite his loose child-rearing style, he's an obviously committed father, and does enjoy spending time with his daughter when he's not working.
Koiwai's best friend in the series is Takeshi Takeda, whose massive height has earned him the nickname Jumbo.