Compassion

Compassion was a remembrance of a human who joined the Remote. She travelled with the Eighth Doctor. During their time together, she metamorphosed into a TARDIS.

Origin
Compassion was originally Laura Tobin, a woman living on the original colony of the planet Anathema. Like all the colony residents, she was or became sterile. They reproduced with Remembrance Tanks. Over the millennia it took for the ship which Anathema was based on to get to Earth, all the colonists were "remembered" using biomass inside the remembrance tanks. Compassion was the result of this process.

Compassion was a member of the Remote, an offshoot of Faction Paradox. After she met the Eighth Doctor on Earth, she travelled with Fitz (whom Laura Tobin had known) and him. She also knew Kode, the remembered version of Fitz. (PROSE: Interference - Book Two)

As Compassion came from a media/signal orientated culture she always wore an earpiece or receiver. To protect her from any 'harmful' signals she might encounter during their travels, the Doctor interfaced her receiver with the TARDIS systems. (PROSE: The Blue Angel) This had unforeseen results, warping her biodata along Block Transfer Computations, culminating in her 'rebirth' as a sentient TARDIS. (PROSE: The Shadows of Avalon)

Life as a TARDIS
With his own TARDIS apparently destroyed in a dimensional rift and the Time Lords seeking Compassion for breeding stock for their own sentient TARDISes, the Doctor and Fitz used Compassion to go on the run. (PROSE: The Shadows of Avalon) Compassion began referring to the Doctor as her "tenant." (PROSE: The Fall of Yquatine)

Afraid of being tracked down by the Time Lords, the Doctor purchased a Randomiser on the planet Yquatine and installed it in Compassion without her permission. The installation and operation of the Randomiser caused her a great deal of pain, to the Doctor's horror, but her systems quickly integrated it and she was unable to remove it. She eventually forgave the Doctor for installing the device, an act which he convinced her was for her own protection. (PROSE: The Fall of Yquatine) Compassion came to be less concerned with her inability to control her destinations, finding the actual travel through the Time Vortex so exhilarating that the destination was unimportant. (PROSE: Coldheart)

Eventually, Compassion was drawn to Gallifrey by the presence of the Edifice just before the Faction Paradox attack and Gallifrey's destruction. Compassion finally rejected her Randomiser, expelling its remains into the Edifice along with her occupants. After the destruction of Gallifrey, Compassion took the Doctor and the TARDIS- now totally drained of all its energy- to 19th century Earth, then took Fitz Kreiner to 2000 so he could meet the Doctor after the TARDIS had repaired itself. She left to explore the universe with a Gallifreyan technician, Nivet. (PROSE: The Ancestor Cell, Escape Velocity)

Appearance
Compassion had coppery-red hair and freckles. Once she became a TARDIS, though she was initially reluctant to explore this new ability, her appearance was a matter of whim. (PROSE: The Fall of Yquatine) Fitz later referred to a time when he and the Doctor travelled the universe in a "stroppy redhead". (PROSE: EarthWorld)

Abilities
Once she became a TARDIS, Compassion's senses extended into other dimensions. She could generate a force field around her and deliver painful electric shocks if attacked. She once accidentally overloaded the brains of four humans trying to contact them via her telepathic circuits, temporarily disorienting them. (PROSE: The Space Age)

As a TARDIS, Compassion's internal dimensions were mapped to her psychology, full of twisted forests and dark, echoing spaces she forced the Doctor and Fitz to inhabit. Her console was a black version of the one in the TARDIS she was familiar with, with a crystal spire in the center. The controls moved constantly, subject to how helpful Compassion felt towards the Doctor at the moment. Compassion used her newly found psychic abilities to influence Fitz's dreams; he admitted to suffering from nightmares when they travelled together, until they reached an understanding. (PROSE: The Fall of Yquatine)

Behind the scenes

 * Compassion is one of the few ongoing characters from Doctor Who to also appear in the spin-off book series, Faction Paradox.
 * Physically, a fictionalised version of Compassion evidently resembled the actress . (PROSE: Interference - Book Two) However, she was described as being "gerbil-faced and chubby". This description fluctuated throughout the series, with Fitz thinking her rather plain but with enough "curves" to make her appealing.
 * Madame Xing in PROSE: Halflife is strongly implied to be Compassion.