Answer:
I'll give you the realistic and fantasy side, fallen angels are part of religion, Christian as in the bible, fallen angels are former angels of God cast out of heaven, down to earth where they do bidding of Satan, (the leader) who started to rebel against God, today fallen angels descise themselves as one of us and temp you, they can be the hot guy on the corner offering you drugs, or simply someone passing by, just like the good angels here, demons r the same thing as fallen angels, they can be in human form or spirit. But I don't think they'd be falling In love with mortals