Compassion

Compassion V was the fifth generation of Laura Tobin created by remembrance tanks. She travelled with the Eighth Doctor. During their time together, she metamorphosed into a TARDIS.

In order to protect humanity during the War, Compassion became the City of the Saved.

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)

Second War in Heaven
During the War, Compassion took part in the first battle on Dronid.

Compassion later met Carmen Yeh in 1758. They began to travel together. During this time, Compassion came to an agreement with the War King to create a "second wave" for the war. Compassion warned him House Lucia was a major event in the near future for him, but did not explain what she meant. (PROSE: The Book of the War)

In an attempt to provide a safe haven for humanity during the War, Compassion lodged herself between her universe and the next. By doing this she was able to change her interior dimensions to create the City of the Saved. (PROSE: Of the City of the Saved...). Fearing being compromised by any one of the hostile War powers, Compassion established a number of additional safe havens for humanity, contingencies or bolt holes, beyond the end of the universe within the bodies of her timeship children and accessible via the Downtime Gate. (PROSE: God Encompasses). Grandfather Halfling was an old friend of Compassion who was present at the creation of the City and helped Compassion create it. (PROSE: The Book of the War)

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) Her freckles were described as being "far too symmetrical to be natural". (PROSE: Warring States) 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; by contrast, she was later able to act as a filter when the Maker spoke to Fitz, adjusting Fitz's perceptions so that he could process the Maker's telepathic message without being driven completely insane. (PROSE: The Space Age)

TARDIS interior
As a TARDIS, Compassion's internal dimensions were mapped to her psychology, full of twisted forests (representing the state of emotion existing beyond the civilised self and Compassion's new awareness of her humanity) and dark, echoing spaces she forced the Doctor and Fitz to inhabit (the Doctor attributed this decision to her subconscious terror at what she had become "punishing" him and Fitz like a woman will scream abuse at the father of her child while giving birth).

When the Doctor and Fitz first entered Compassion, they found themselves in an ornate corridor decorated with portraits of the Doctor's friends, family and past selves in varying artistic styles, progressing to another corridor with doors leading off to Compassion's subconscious — with such contents as Awful Truths, Hopes for the Future and That Dream About Fitz, the last featuring Fitz's voice screaming about something — along with a narrow bridge over the dark chasm of her unconscious leading to the console.

Her console hovered over her subconscious blackness on a stone platform, the console itself being completely black and possessing harsher angles than the Doctor's original TARDIS, with a large crystal as its centre. The controls moved constantly, subject to how helpful Compassion felt towards the Doctor at the moment and her own uncertainty about what she had become. (PROSE: The Shadows of Avalon) 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

 * Physically, a fictionalised version of Compassion evidently resembled the actress Nicole Kidman. (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 Halflife is strongly implied to be Compassion.