Chinese Title: Tian Shi Jin Lie Qu
Setsuna Mudo is a problematic delinquent high school student, who
is in love with his sister. To add to his problems, he is also a
reincarnation of a fallen angel, which gets him caught in a messy
war between heaven and hell.
Angel Sanctuary is one of Kaori Yuki's most celebrated works. Among
other things, it discusses the morality of incestuous
relationships, the nature of God, and other philosophical
topics.
http://en.wikipedia.org/wiki/Angel_Sanctuary
One of my favourite manga, haven't watch the anime, it seems there
are some charcters in it that are not present in the manga.
Fundementalist Christians can forget reading it. 
