It sounds nearly like the name of a thriller novel. Gone Girl, Girl With the Dragon Tattoo, Girl With a Tooth in Her Head. By which I mean she has a perfectly formed adult tooth, roots and all, right in the middle of her brain. Like all of these sorts of magical creatures, you will meet her late at night and when you are alone. She seems to prefer parking ramp elevators, especially if it is near a dentist's office. Another reason to fear the dentist.
She is a little girl in a frilly dress and pigtails and she will open up the top of her head and show you the tooth. If you react in horror she will jump on you and pull out all your teeth and eat them. If you stay still she will pluck the tooth out and hand it to you. Again, to recoil leads to forcible removal of teeth. Whatever you do, don't freak out.
If you simply take the tooth she will snatch it back and all your teeth will rot and fall out of your head within 3 days. What you need to do is offer her something in return. Even if it is just the tiniest bit of yourself. Then she will take the part of you and let you have the bloody tooth. When you look for it in the morning it will have changed into a $50 bill.
I know all of this because I met her one night. I gave her a bit of my skin peeled from a callus on my hand. Though this seemed the best possible answer at the time, now that I think of it, a magical being now has my skin. But I'm sure nothing will happen..... Right....?