The Corsair

The Corsair was a Time Lord and friend of The Doctor on Gallifrey.

He/she is known to have gotten a tattoo of Ouroboros applied to every regeneration during his lifetime, and was also said to have changed genders, presumably through regeneration, more than once. The Doctor described her as a "bad, bad girl."

The Corsair died after storing a psychic message in a hypercube asking for help when he arrived on the House entity in a bubble universe. House used The Corsair's body to patch up Auntie and Uncle and later used his/her hypercube to lure the Doctor to the bubble universe, under the pretence of The Corsair still being alive and needing the Doctor's help, to try and capture the Doctor's TARDIS. (DW: The Doctor's Wife)