Anime Slenderman?

she stalks, she kills, she's scary as hell... it's kinda like slenderman.
(except that she's not tall, she doesn't have wierd arms, and she actually has a face 0.o aaannndddd she doesn't wear a suit <-- could be fixed :P )

not the best answer i know, but honestly, this is like the "human" girly version of him. 'cuz when i say Gasai Yuno can make u disappear, i'm dead serious.

Lucy from Elfen Lied, she can kill in seconds and has long invisible arms (vectors) Although I can't imagine her living in a forest, etc