This answer contains spoilers… (view spoiler)[I thought there were several hints along the way that indicated there could/would be something going on between them beyond the mentoring relationship. Another character observed that Warden was generally considered to be good-looking for a Rephaim (which kind of also explains why he was blood-consort even after betraying Nashira previously....), Nashira's suspicion at smelling the human on his sheets, questions about flesh crimes, and the water/oil/fire discussion between Paige and Warden all hinted at this possibility. But the real clincher was during the memory flashback to Nick's confession/rejection - the aspects of love that Nick and Paige discussed perfectly described the relationship forming between Paige and Warden. (hide spoiler)]
This answer contains spoilers… (view spoiler)[I remember at the beginning of the book I totally expected them to have some sort of forbidden love-thing, but it didn't really show until the end. The build-up was really (really!) slow, but I think it made it that much more powerful. I was shocked to hear about Nick, though! Totally didn't see that one coming! (hide spoiler)]
This answer contains spoilers… (view spoiler)[I felt pretty weird about the kiss as well to be honest - he's her 'keeper' and so much older than her, and while he protects her from certain things he's also 'feeding' on her and putting her in dangers' way... I saw it sort of coming but I'm not sure how I feel about them as a pairing to be honest! So you're not alone in thinking that :) (hide spoiler)]
This answer contains spoilers… (view spoiler)[I was somewhat taken back by it, but not that much really. I could kind of see that Warden liked her, but it was hard to read Warden's emotions given how Rephs don't seem to show and/or experience them the way humans do. Paige was in love with Nick, and was still heartbroken, but it is not impossible to love two people at once. The whole idea of everyone only loving one person at a time is romantic, but somewhat idealistic. So, it is not at all strange to think that Paige could have feelings (or even be in love with) two people at the same time. I could see her attraction to Warden. The romance that budded after the kiss felt a bit shocking. Even forced. I had assumed her attraction was mostly lust, and that his was romantic. I was, apparently, wrong. Which is the only part that shocked me. (hide spoiler)]
Anab Lol wot how could you not have foreseen that? Haha I was kinda surprised somewhat because I thought they wouldn't kiss as well until later on.