There is a legend as old as time on Gallifrey, it is said the the perfect someone is created for those deserving of them. Not born but created by the Vortex, they would be TimeLord but not of another TimeLord. It was only a story, a story The Doctor was told as a child. He never believed in the stories at least not until she was created just for him. The TARDIS gave him the one thing he had always wanted. A Familiar. A Bond that could never be broken.