Narue no Sekai

Kazuto Iizuka is your average 14-year-old boy. That is until he has a run in with another student at his school, Narue Nanase, when she saves him from an attacking alien creature. During the ordeal Kazuto discovers that Narue is actually half alien herself and that her father is part of a galactic exploration team. With the encouragement of his friend Masaki Maruo, Izuka asks Narue out to on a date. Narue is reluctant at first, but after Kazuto assures her that he is not bothered by her alien heritage Kazuto and Narue start their relationship as boyfriend and girlfriend. However, like any couple they have to deal with their fair share of problems; such as Narue's alien sister Kanaka and Hajime Yagi, Narue's classmate and ufologist who doesn't believe Narue is really an alien and thinks that Narue is lying to get attention.

(Source: Wikipedia)