Chihiro Aoi

A mysterious boy who had celebrated Sakuya's birthday and then vanished without a trace afterward only to return later as a student at Shiina's school. He lives with his uncle and aunt. Since ch.27 or so of the manga revealed a very tragic story why Chihiro have such a personality (being bad to Shiina) and moved to Shiina's place. (This relates to him and his previous girlfriend, Sakura)