The Corsair

The Corsair was a Time Lord described by the the Doctor as "one of the good ones." His signature emblem was a classical depiction of the mythological Oroborus, a snake devouring its tail. According to the Doctor, he had the emblem tattooed on his body after each regeneration, even when he regenerated as a female. Without it, the Corsair didn't feel like him/herself. In his last regeneration, the tattoo appeared on the inside of his left forearm.

The Doctor described one of the Corsair's male incarnations as a "fantastic bloke" and one of his female incarnations as "a bad girl!" The Corsair was presumably killed by the entity known only as House, a disembodied being that fed on artron energy it harvested from captured TARDISes. The Corsair's left arm had been stitched onto the composite humanoid creature being known as "Auntie" that served as House's slave. The other composite humanoid creature slave "Uncle" received "the spine and the kidneys." The loss of these organs was presumably lethal and the Corsair went the way of House's numerous other Time Lord victims.

The final incarnation of the Corsair was described by Auntie as "a strapping big bloke." The Doctor discovered his fate after being contacted by his hypercube and following its psychically encoded distress message to House. (DW: The Doctor's Wife)

Behind the scenes
On his blog, Neil Gaiman stated that before he began writing the first draft, he wanted to make sure the idea of the Corsair was okay with Steven Moffat before he became fixed in the story, so he sent him an email with a piece of dialogue between the Doctor and Amy in which the Doctor talks about the Corsair. The Doctor explained that the Corsair did not have a name and used to travel, exploring the limits of time and space. The Doctor said that when he was twelve he asked the Corsair if he could travel with him and act as his "assistant", but the Corsair just laughed. Steven Moffat replied that he wanted the Corsair to be less like the Doctor because the Doctor "does what he does for reasons too vast and terrible to relate".