Ren Aoi

Ren is an angel, who had saved Nikita from death in the same crash which his parents died from. Since this was changing the destiny of a human, which is forbidden for angels to do so, Ren is punished by God to live on the Earth temporarily neither as an angel or human. By the end of the episodes, it was time for her to leave Earth, as she had served out her sentence.

(Source: Wikipedia)