Spoilers for Journey to the Center of the TARDIS
He is young and she is old when they first meet.
He spins around her console, runs his hands across her many levers and buttons, crying out with joy and reckless, wild energy. "Now, you," He says, and his mind twines through her, into her soul, shining with youthful curiosity and a thrumming kind of madness that no other Timelord has, and she just knows-
"You," he continues, "are the most beautiful thing I've ever seen."
So she steals him and they run away.
For countless years(at least she thinks it's years, time is all wibbly wobbly after all, not a straight line) they run together. Sometimes he brings in strays-his Granddaughter, a couple professors, a journalist, aliens and humans and reckless creatures that always get her Doctor into trouble.
But then sometimes they go home, and he is always angry. He storms around yelling that things aren't like they used to be, like they should be-
So she hums to him and sends sweet dreams of running under silver-leaf trees before he'd been forced to see all of space and time, before he went mad in the most wonderful way possible. She had found these memories their first flight together, and draws them out whenever he needs comforting.
While he sleeps, she listens to the singing of her sisters and the pounding of drums from deep within the heart of time itself.
Then they fly off again and time is more wibbly wobbly than usual. It's wild and reckless and mad.
But then her thief is suddenly so sad. He pets her console and whispers something about war and time and terror, and she doesn't understand his words, but she understand that black darkness in his mind and she shudders because she knows-
He pleads with her, says they have to go home, even though she doesn't want to, because she knows, oh she knows-
In the end, she does take him back. Because it's where he needs to go.
She's burning.
The console is on fire and somewhere the library is too, everything is fire and ash and crumbling dust-
Her sisters are screaming-
(Dying?)
Time explodes across all of space in searing heat and it hurts-
And then her thief is running through her doors and whisking them away and they are running like they never have before. Running not because it's fun, not because they're seeking adventure. They're running because they're terrified and this is the end of everything and anything and the planet is burning and-
His eighth body dies, covered in burns and ash, and screaming in agony. She soothes him through the regeneration as best she can, but the Time War hurts her too.
His ninth body wakes violently, vaulting upright and grabbing onto her console so tightly.
His first word is, "No!" and his first action is to slump to the floor and sob while she takes them deep into the vortex.
He is so broken, her thief.
Now she finds herself bringing for those happy memories of silver leaf trees and orange skies more often. He needs them. He rages and screams and pounds his fists against her doors, begging her to take him home.
But she can't. Because they can't ever go back, and she doesn't want to think about all of that time-energy and the terrible fear she'd felt when the Time War ended.
She hums Gallifreyan lullabies in his mind to quiet him, and sometimes she creates an extra room full of memories that looks exactly like his favorite place back home.
He sits in there, hugging himself, whispering like a child. Sometimes he reaches out to her with his mind(That beautiful, wibbly, wobbly mind) and they hold each other with their souls until they both aren't screaming quiet so much.
He finds new strays.
At first she is jealous of them-the pretty blond lady and the cocky boy. The quiet one and the red haired one and the other red haired one and the pretty one. But they help him. They heal his breaking hearts and mend the little pieces of him. His soul sings brighter to her now, when he is with the humans.
But it's still her job to stop his nightmares before his companions see all of his ugly, black darkness.
And she doesn't mind in the slightest, especially when he lets her pick where they really need to go.
For many more years, they run together.
Sometimes it's with strays and there is always the, "bigger on the inside" conversation and she purrs while her Doctor laughs. If only they knew how much bigger she really is. Her thief knows how infinite she is, but that's their secret.
But other times, they're alone and he sits in her console room playing with his bowtie or his glasses and he talks to her about Gallifrey and memories and madness and how very grateful he is that she is there with him.
She clicks and whistles to let him know that she's grateful for him to. She hopes he gets the message.
He is so old again. She is older still.
And she's breaking and she's burning, and she's tired.
He knows. She knows, he knows. She can always tell.
He walks hand and hand with this newest stray(that she doesn't really like) and he's so lost and sad. "She's just always been there for me, and taken care of me and now it's my turn and I can't-"
Silly, silly old, Doctor.
She shows him how to help with the fragil-ness of human skin, and he spins and claps and drags his stray away.
But she hears the words whispered to her, only for her, inside his mind, Thanks, old girl.
And she hums, and lets him run. Because that is what they do. Forever and ever until they can't anymore: they run.
He's her thief and she's his TARDIS.
And they always take care of each other.
Always.