The Corsair
The Corsair was a Renegade Time Lord described by the the Eleventh Doctor as "one of the good ones", despite this, many Time Lords disapproved of him/her. S/he took on multiple genders throughout his/her many regenerations. S/he was known for his/her manipulative smile. (REF: The Brilliant Book 2012)
His/her 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. At its larger, the tattoo was multi-coloured and covered his/her back during her third incarnation. It was at its smallest during his/her fifth incarnation, at the size of a ten-pence piece, on his/her upper thigh. (REF: The Brilliant Book 2012)
The Corsair owned a TARDIS that usually took on the form of a sailing ship, even when it was not practical, because s/he thought it was cool. 
Despite his/her renegade status, the Time Lords used the Corsair to perform various tasks which they did not want to be connected to, much like the Second Doctor. It was thought that this why s/he adopted the name 'Corsair', as it was a term used to describe legitimate pirates. Amongst her/his missions for the Time Lords, s/he stole the Callisto Pulse from the Callistan Kleptocracy. (REF: The Brilliant Book 2012)
Though it is not specified, this implies s/he was an agent of the Celestial Intervention Agency.
Although s/he denied ever having fought the Daleks, she was spotted in her seventh incarnation on Clarkor Nine when the Dalek scout ship arrived. It is highly likely that she was involved in those Daleks having their suction-cups and manipulator arms removed in the night and fused into a shape resembling a Skarosian profanity. (REF: The Brilliant Book 2012)
S/he had visited Earth several times. He was worshipped as a god by the Assyrians until he became bored and departed with the sacred temple cat. (REF: The Brilliant Book 2012)
S/he never had companions, although was known to keep pets such as cats and parrots. (REF: The Brilliant Book 2012)
The Doctor and Corsair had several adventures with one another, getting drunk together in the Corsair's fourth, fifth and eighth incarnations. Each time, the Doctor swore never to do it again. Twice, they woke up in jail and other time they woke up in the Bank of England vaults. (REF: The Brilliant Book 2012)
S/he was formally censured by the Time Lords after s/he may have been linked to the theft of the Portrait of Rassilon. This was revoked by Lady President Flavia, perhaps due to the Corsair's smile. (REF: The Brilliant Book 2012)
The Corsair's final adventure was when he was working on the Time Lords' Fourth Universal Survey. 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. He was in his ninth incarnation, a stroppy male, at the time. (REF: The Brilliant Book 2012) The Corsair's left arm had been stitched onto the composite humanoid being known as "Auntie" that served as House's slave. The other composite humanoid 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. (DW: The Doctor's Wife)
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".[1]
Gaiman expanded on the history of the Corsair in the The Brilliant Book of Doctor Who 2012 in which he indicates that the Corsair had a total of 9 regenerations, several of which were female.
