Tenshi no Tame no Shohousen.

Summary

Sakuya, a 3rd division archangel has been watching Shirou, a human, through the fountain of life that separates Heaven and Earth for quite some time. As his halo has "fallen" into the human world he decides to retrieve it in order to meet the one he's been watching, regardless of the rules he's breaking. He'll break all the rules to be with the one he loves who has a dark shadow cast upon him... (Source: MU)