Evja

 was Grand High Seer of the Unon, and Observer of the Seven Realities. She brought Captain Jack Harkness into a sub-dimensional void as "insurance". Jack got her attention, and asked her what the future without the Doctor, just the Unon in charge of time, was like. When she did this, she freed Jack and helped save the Ninth Doctor from being attacked from behind. With no way of the Doctor stopping the Perpetual City from overloading, she joined the Unon on another world as a "fresh start", the Doctor asking her to promise that the universe would be left to look after itself. (COMIC: Weapons of Past Destruction)