She wasn't even his, to begin with.
She could, quite easily, have spurned him, this invading virus tinkering amongst the wires of her body, and spat him back out on the plains of Gallifrey he so desperately rejected. But there was something about his recklessness, about the way he treated her, from the first, with care. Some childish rebel, so much younger than her, and with life still pumping violently through his twin hearts. It had been too long since she tasted youth. She was eternally tainted as second-hand, an older model, the word said with such distain and superiority.
He didn't care. "Come on, old girl," he said, with a smile and a certain fondness. "Neither of us want to rot here. Come on."
So she went on. Learnt his name, learnt his mind, and showed him the stars.
* * *
The TARDIS became his constant. The Doctor's companions came and went, and even his bodies were fleeting, but she was always there. He liked her as a friend — someone who isn't always reliable, shows up late to parties and the like, but never answers back and tries her best. That, he couldn't deny.
He developed a habit of petting her, and found she responded to it like a cat being scratched behind the ears. A guttural hum could easily be mistaken for a purr. "You treat this ship too well," Romana had said once, and who had, on more than one occasion, tried to convince him to trade it in for something that actually worked. "You shouldn't stroke it like that. Poor thing sounds like it's coming when you do that."
The Doctor had stopped for a while after that, and the TARDIS became more temperamental than ever. When Romana had left him — as they all did — he put his palm on the bulk of the console and ran his fingers up and down it until the walls shivered.
* * *
What he liked about her best was that she knew him (too well, it might turn out, but for the time, well enough). So when he woke up once — in his eighth incarnation, which was a tad reclusive — and found the library had moved itself to the console room, he couldn't help but grin.
They travelled together, just as two, for a time, because humans confused matters and got in the way and put too much emphasis on emotion. The Doctor could call her beautiful and not have to go into a rambling oration about what exactly he meant by the word. She flirted back in her own way, flighty enough to drop him in the middle of the St Valentines Parade in New Time Square. He laughed, patted her fondly, called her pert, and left it at that.
In the aftermath of the war, he broke the needle off his gramophone because it happened to be in his hand one day. He sat in his armchair, biting the back cushion to stop himself crying, and the listened to the TARDIS replicating the smooth, warming strains of Julie London. She tried too hard, until he shouted at her to stop, slamming his fists against the hard wood of the console.
(He apologised later, kissed the glass engine, and begged her to play it again.)
* * *
She seemed to shed her skin, reborn like a phoenix, this bronze and shining thing. There was something organic about the way she looked, something living and lived-in. Not homely like how she had been, the Doctor admitted, but majestic, and like a real ship. Rusted around the corners like a well-travelled junk boat.
"You've been re-decorating," he called up to the rafters. The TARDIS let out a quiet drone that begged his approval. "Oh yes," he said, running his knuckles over the rough walls. "Very good. I'll have to dress to match."
He found a leather jacket stuffed away in the corners of the wardrobe, tried it on (a little large, but he was lankier now, apparently underfed) and pulled on the lapels. "What do you think?" he asked her. "Bit of the old this-is-the-captain-of-your-ship-calling, maybe?"
The TARDIS said nothing back of course, but bathed him in an amber glow, and the jacket stayed on.
* * *
The problem with regeneration was that inevitable personality change. He became afraid, terribly afraid that one day — just as he had once found humans an irritation — he might lose patience with his old, worn out ship.
Not this time, though; this time came energy and passion and a strange new sensuality that he couldn't help but find intriguing. Rose clung to his arm like a girlfriend, and Reinette pushed him against the mantelpiece, kissing lips she barely knew. The Doctor looked at himself in the mirror in his room once, naked, trying to puzzle out the attraction. He could feel the TARDIS watching him, given away by the prickling sensation upon his bare back. "I don't know," he murmured, cocking his head to one side. And then, looking up: "How about you?"
A faint breeze stirred around the room, tickling the skin of his thighs and chest. It felt good; warm.
"Yes, that's quite enough of that," he said, not entirely berating, and pulled his trousers back on.
He developed an astonishing sense of taste as well, which was fresh and fascinating. He tried this out on his girls too, licking Rose's neck once when she hugged him. It made her giggle, a high-pitched, embarrassed sort of giggle, and she hit him lightly on the shoulder like he shouldn't be so immature. Reinette was more appreciative when he ran his tongue from her wrist to the crook of her elbow. She stroked the side of his face, presented her other arm, and told him to do it again.
Out of curiosity, looking a little guilty over his shoulder, he knelt down and licked the curved rim of the console. The TARDIS tasted of metal and plastic, something like Gallifrey and something like him. She seemed to shudder under his fingertips. Made bold by her response, the Doctor kissed a gentle trail up the side of her, darted the tip of his tongue against her glass, took the head of a lever in his mouth and sucked carefully, softly, all the while caressing her.
He did it because he loved her, and because she was all he had left. They only had each other. (Rose did not walk in to accuse him and remind him of her constant presence. She had no tact, only ignorance, and he was glad of it for once.)
The walls of the ship creaked and the place shivered all over, a slow and thankless orgasm, if it could be called that. The Doctor wiped his mouth on his sleeve, patted her awkwardly, and smiled. The air inside had become thick and hot; he hadn't noticed, but loosened his tie now.
He went to Rose's room that night, and, tight and pent up, fucked her briskly in her bed. His shirt was still buttoned up and his trousers hung like a burden around his knees. He came inside her with a hollow, earthy sound, pulled out, and rubbed against her with his fingers until she bucked and cried out his name.
"Thank you," she said afterwards, kissing him. It wasn't so much the sex he enjoyed, but the reciprocity of it.
The TARDIS was cold for days afterwards, and the Doctor had to take to wearing his coat inside.
"Don't you have central heating or something?" Rose said, shivering and naïve.
"No," the Doctor growled, and kicked the bottom of the console roughly, hard enough to hurt himself too. He deserved it, he supposed.
* * *
Once, twice, too many times, he thought he'd lost her.
After he saved her (she saved him) from the Bitter Pill, he went to bed with Rose and got their reunion out of the way first, staying with her until she fell asleep. Then he lifted her head gently, slipped his arm out from underneath it, and walked naked down the corridors of the ship, stroking his fingers along every wall. He lay down in the console room, curled underneath the heart of the TARDIS itself, and kissed every wire, every bolt, everything he could touch, whispering promises he knew he could only try to keep.
"What are you doing?" asked Rose. He could see her feet, ankles, and the bottom of her dressing gown through the cord maze.
"Go back to bed, Rose."
The feet shuffled awkwardly. The nail-varnish on her big toe was chipped. She coughed, from somewhere above. "…Have you got no clothes on?"
"Rose, you're tired. You've had a hard day. Go to sleep."
The feet didn't move for a moment, then slowly, reluctantly, trudged away until he couldn't see them any more, but could only hear the pad of skin against metal.
* * *
She didn't remind him of home — he didn't want that anyway — but gave the allusion that there was a home to return to. He never quite believed it. No amount of gloss could buff over the fact that he was the last of his kind and she the last of hers. But he clung to that sense of hope she gave, and thought of her engine as a beacon, a pillar of light calling out into the distant corners of the universe for a sign of life.
(He didn't have to go too far to find that acrid taste of reality. Across the room, Rose's smile dripped with it.)
* * *
Rose left, of course, as they all did.
The Doctor sat inside his vast ship, his knees hugged to his chest, and talked about nothing in particular. He wasn't talking to himself, not even to the TARDIS, but just for the sake of words. She couldn't give him that. He habitually conversed with himself, now that he was alone again, and laughed at his own jokes, and cried for other peoples' mistakes. He was his own judge, his own jury, and the TARDIS provided an arena for him to verbally punish himself.
He leant over the control panel sometimes, jerking off furiously with his eyes closed, stroking her with his thumb. He would always come first, and only if he was in the mood would he bring her off with his nimble hands and lithe tongue. Sometimes he loved her for it, called her his constant, and sometimes he was ashamed, hanging his head in his hands and apologising to the air.
It was simpler, perhaps, when she was just a vessel. But the Doctor didn't like to think on it too much.
* * *
She showed him the stars, and all he did was point to the horizon, saying, "There next, girl. There next."