The Corsair

The Corsair was a Time Lord, described by the Eleventh Doctor as "one of the good ones". The Corsair took on different sexes throughout many incarnations. The Doctor noted that when the Corsair was female, "she was a bad girl."

Biography
The Fourth Doctor once said to Romana that he should meet up with the Corsair sometime; he described the Corsair as both male and female in the same sentence, implying one of his regenerations had already been female. (PROSE: Shada)

The Corsair had the symbol of the Ouroboros tattooed on each of his or her incarnations and also imprinted on his hypercube. The final incarnation of the Corsair was described by Auntie, a composite humanoid servant of House, as "a strapping big bloke." She was a good authority on the matter; after all, she had the Corsair's left arm stitched on to her torso in place of her own, while 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.

The Eleventh Doctor discovered the Corsair's fate after being contacted by a hypercube and following its psychically encoded distress message to House. (TV: The Doctor's Wife)

During one adventure, while pushing a heavy crate, the Eleventh Doctor said that if he kept up this habit, he would develop big arms like the Corsair. (GAME: The Eternity Clock)

The Brilliant Book 2012
According to The Brilliant Book 2012, a book that contains non-narrative information, the Ouroboros tattoo trademark of the Corsair — a snake eating its own tail, symbolising eternity — moved around the Corsair's body with each regeneration.

Amongst his secret missions for the Time Lords, the Corsair stole the Callisto Pulse from the Callistan Kleptocracy.

He visited Earth several times and was worshipped as a god by the Assyrians.

The Doctor and Corsair had many adventures together, getting drunk in the Corsair's fourth, fifth, and eighth incarnations.

The Corsair was formally censured by the Time Lords after they may have been linked to the theft of the Portrait of Rassilon. This was overturned by Lady President Flavia, perhaps due to the Corsair's smile.

The Corsair's final adventure was when he was working on the Time Lords' Fourth Universal Survey. He was in his ninth incarnation when House killed him.

Other matters

 * The Corsair was not the only Time Lord to bear a snake tattoo on his arm. Whereas the Corsair tattooed an ouroboros on each of his new bodies, the Doctor's third incarnation came with a tattoo of a snake in the shape of a question-mark on his right arm. It was actor Jon Pertwee's own tattoo from his Royal Navy service during the Second World War. (TV: Spearhead From Space)
 * On his blog, The Doctor's Wife scriptwriter Neil Gaiman stated that before he began writing the first draft, he wanted to make sure the idea of the Corsair was okay with series show runner 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 Pond in which the Doctor discussed the Corsair. In this draft, 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 had asked the Corsair if he could travel with him and act as his "assistant", but the Corsair had 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".
 * According to Neil Gaiman, Russell T Davies liked the idea that the Shopkeeper from Sarah Jane Adventures episode Lost in Time was actually the Time Lord known as The Corsair although no in-scene references have been made.
 * The Corsair, being the first confirmed example of a Time Lord changing genders on regeneration, was frequently cited in media and online discussions in 2013 as to whether any future incarnation of the Doctor should be played by a female.