First of all, the story was absolutely genius. This was a very unique retelling of the Persephone myth. The mythology was written so well; I truly have never read anything like this before! Originality is very important for me, and this one scored an A+ in that department.
I really enjoyed all the characters too. Nikki was an admirable protagonist; while she definitely did have lapses in judgement sometimes, she was strong in her decisions to protect others, even when it meant sacrificing herself. I am also torn between the awesomeness of both Cole and Jack. While you could say Cole is the "bad boy" and Jack is the "nice guy", I don't want to think of them in those stereotypical ways, since they're better than that. They are both very complex and amazing in their own ways. I'm glad I'm not Nikki, because I could never choose.
Plus, the ending?! Like, WHAT?!?!?! I really need Everbound!
Overall: REEEEEAAAAADDDD IIIITTTT (Sorry for the excessive caps, but the situation called for it.)