Seien Tenshi Eleanor

Protagonist awakens in a forest and encounters a battle between a monster and a nun in torn clothes. In addition to these surprises, he also doesn't remember anything but his name, Neon. Judging from his attire, he assumes that he's part of the local Christian school. The people at a seemingly all-girl school accept him, but they don't seem to know who he is either. While working there, Neon tries to piece together his past, as well as help the strange angelic woman who saved his life.

[Edited from White Masq]