Mysterious Girl (Harry Potter Fan Fiction)

She can apparate into and out of Hogwarts when it should be impossible.

She can randomly apparate to wherever Harry is when he's in trouble, though she doesn't know who he is.

She has caught Dumbledore's attention as an exceedingly powerful witch, yet she seems to be unaware of it.

Abused, regularly beaten and treated like dirt on the bottom of her Master's shoe, Robyn (as she often called herself) only had the letter of an unknown person to shred some light on who she is. But when she finds her life is twisted with another's whom she is to protect, what will happen?

No one seems to know anything much about Robyn, so who is this mysterious girl?