As dumb as they come.

A pretty face
with wide open brown eyes
starved somewhere near dead.

She's smart enough, manipulative enough to become whatever she wants to become. She chooses to be an idiot, a likeable idiot. You tell a joke, she'll laugh, mouth wide open, gums showing in a grotesque manner; one eye open, one eye half closed, her laughter being pretty much the protrait of a mentally challenged kid. Someone you can trust. What could a retard possibly ever do to you, but follow you around like a dog laughing at everything you say?

She's just loading up on diffamation ammo. She won't say something straight out of order.
She'll analyse you for weeks or months, she'll learn just enough about you to say gruesome things that seem true. Say you're a guy who is constantly trying to seem cute. Desperate attempts, you might say, to pick up a girl (say, maybe two), but you never really mean no harm, still she knows to any other third party in the story, it would seem reasonable that you're acting this nice to overcompensate for something, being a closet rapist.

And she'll say it, she'll spread the word to her friends, to your friends. Say that's why she stopped talking to you, you tried to attack her and they will all believe it.