You mean besides Renard’s epic betrayal? Knowing these writers, it could be anyone. But of everyone, the three most likely candidates for me are:
Adalind – I find her most likely out of them because although she may love Nick (or think she loves him, it could still be magic weirdness), she also has two small children that she will always put before anything or anyone else. They’ve proven to be her pressure point in the past, and they could be again. If it comes down to Nick or her kids? No contest.
Juliette/Eve – I really don’t want this to be the case, but Julieve is still such an unknown in so many ways. Based on the end of last season and the start of this one, I want to think she’d come down on Nick’s side no matter what. But there’s a lot of pain, grief, and conflict swimming around in there. It’s just hard to know for sure.
Trubel – She seems to me the least likely to betray Nick, but…well, she kind of has before. She came to retrieve Juliette without talking to Nick about it on behalf of HW at the end of season four. She seems to do what they tell her to, for the most part. So what if HW told her to take Nick out? I’d like to think she’d balk, but we’ve seen less and less of her…they brainwashed Juliette’s personality away, who knows what they might have done to Trubel in all that off-screen time?