Phantom Willow is a form of Willow the Wolf, you can only see her when Willow is Either Extremely depressed or when she's just about to cry.

Her appearence:Her right hand is missing,Wires exposed on her left leg,Large hole in the center of her chest,Left ear missing,right one half torn off,Endo-Skeleton mouth hanging out the Right of her mouth

She's a bit more interesting than Withered Willow and Toy Willow.

I'll add more information soon.