Hiro is a young man who was once Shōko's boyfriend, but who inexplicably left her to take care of Karada, who he seemingly greatly cares about. He also studied abroad, with Shōko, which is how he met her.
It is later revealed that he isn't Karada's older brother, but a relative's son. He only met her for the first time at her parents' funeral, and prior to that he never thought of her as his sister. He thought that he would return to America right after the funeral, but he couldn't leave her alone.
