The Rani

The Rani was a renegade Time Lady who travelled around the universe in her own TARDIS and caused mayhem and destruction wherever she went. The Rani herself isn't all that threatening, she has been beaten in hand-to-hand combat by the Doctor himself several times, but she does have a nasty habit of allying herself with all manner of twisted creatures, such as three-eyed bats, Cybermen, Autons and even giant Space Worms that eat planets.

The Rani has her own TARDIS, but its really weird. Even the Master, who is known Galaxy-wide for his insanity, commented on how twisted her TARDIS is when he was allowed inside. The control room is rather like the control room of the Doctor's TARDIS, except there are brains everywhere. It's really, really odd.

The Rani herself disappeared after the end of the Time War. Whether she took part or not is unconfirmed, however she can assume that she was on Gallifrey at the time of its destruction since she loved a good battle and she hated Daleks. However, she did have a habit of slipping out of trouble, so there's a chance that she's still out there somewhere...