I'm surprised no one noticed this before, but when Haley is talking in gibberish to Elan at the restaurant scene, she says (translated) "I'm not exactly what you call--" and never finishes. Maybe the next word was going to be 'human', indicating that she is half celestial!
Really, why is it that no one has suggested this before?
Seriously, people have noticed before. And they have proposed outlandish theories like Haley being not human based on that one line. This isn't anything new.