The Master

"Then you should have phrased it better." The clever almost-Time Lady responds with a sly grin. Oh please, she used to run from plenty scarier things then this man and she'd been a toddler back then. "How I came to be? Oh, you know....mum and dad were doing the birds and the bees inside the TARDIS years ago - mid-flight through a time vortex. Nine months later, out popped little old me."

So, she was the daughter of two companions of the Doctor and being conceived inside the time vortex made her very special indeed. One of a kind. "That good enough for you?"