UPDATE: We've spent an hour or so trying to feed the runt. She hasn't taken to the bottle, or syringe. I've been looking all over the internet, and everything points to FKS. If anyone has experience with an underweight kitten in their litter, I would really appreciate any related advice. I'm going to try to give her more alone time to nurse from mom the next couple says and will probably take her to the vet tomorrow. I'm hoping that everything turns out alright, and I'll keep you guys posted on her condition.
She is the black one on the bottom