Kotarou has been noted as being 10 years old. But whether he is Jirou's brother or not, that's another thing. Considering he looks a lot like Alice Eve (Jirou's lover >_>).

He very cute, perhaps to the point of being annoying, but Jirou makes up for that.... *Ahem* In the first episode while on board the ship to Japan, Kotarou comes across a little girl, named Chan, who he later gets to play with a little. In Episode 3, they meet up again for a time. However do to a Kowloon's control, Chan is forced to help kidnap him, and is later killed. Jirou states that he will be fine, when Mimiko asks about him. But boy, is he energetic. x.x