The Observer

any sign of interference from The Fallen, people of his kind who have broken 

their most important rule: never interact with humans. But when Gabriel 

becomes fascinated with a human girl, Alyssa, it isn't long before he justifies 

breaking the rule himself.

His forbidden association with her doesn't go unnoticed. The Fallen
seek to recruit him and The Enforcers, a separate caste of his society
who ensure punishment of those who break the rules, force him to
hide--all the while trying to combat a centuries-old plot by The
Fallen to destroy humanity and claim Earth for themselves.

Spending most of his time on Earth, Gabriel realizes he's acting more like a human than an Observer, and he must make a choice: sever all ties with his people, or lose the woman he loves.