Erin.

I hope they don't go on too long with this storyline, I know the character has past issues that have led her there and her mom is absolutely toxic, but she's been a strong personality throughout and I think she'd fight back sooner rather than later.
I wonder who will help her snap out of it, I could see Hank calling in Olivia to talk to her.
It was cool to see Atwater get called back onto the unit, shame Burgess got bumped back down though as it was good character progression for her, I wonder if, with Erin "quitting" Voight will give her the spot. Of course now she is engaged to Ruzek she could end up getting killed as is the way on these type of shows.
Voight is such a unique character, this week he said that the corrupt cop who wanted to be him didn't really know who he was, it's hard to pin exactly where his line is.