So you see Estrell is a goddess, looking like an angel of some sort? Well I have a theory on that, and why she might not be as benevolent as she seems!
You see, demons are supposed to be tricksters- I know, I know... But if there's Estrell, then there must be an opposite of her that is VERY mad that the demons/devils lost... I think that the big villain is disguised as Estrell because gods don't just show up and talk to mortals- they think themselves better than mortals. Not to mention that the light side isn't always as good as it seems! So, what if the villain is disguised as Estrell, or that there is no Estrell at all? I have another theory too- Assuming it actually is Estrell, Estrell might not be showing her true form and her intentions. She just showed up, told Xin that she will fulfill a destiny, and dipped... And she seems too good and innocent to be the good one.