The Girl Who Stayed.

"Run."
That had been two hours ago. He didn't know why he followed her. He never knew why. Only who.

Suddenly she stopped, and he nearly crashed into her, but she spun out of the way, laughing as she did so. He took a moment to catch his breath before straightening up and opening his mouth to speak.
"You," she interrupted. "You want to know why we ran, Doctor. Isn't that what you normally do? Run?"
"Why, yes, there is a lot of running, but...why?" He replied, smoothing his jacket.
"Why? You never do know why, do you?" He was silent, and she grinned like a Cheshire, her raven black hair floating on the wind around her, in a non-existent breeze.
"Only who." His words were quiet, serious. She knew what he was thinking.
"You don't want another one. We only bring you sadness, in the end, isn't that right? But isn't it worth it, to be happy?"
"You say 'we'."
"You never know why, Doctor. Only who." She shook her head, her smile becoming a sad one, the smile of someone who had lost much, but still aimed for happiness.
After a moment, he spoke up again. "What were we running from?"
A growl echoed throughout the clearing and the ground shook with a rumble.
"Spiders." She grabbed his hand again, and this time, he laughed with her, running with her towards the TARDIS.
When they reached the doors, he snapped his fingers, grinning like a madman as they swung open. They stepped inside and he turned to her.

"Vivienne Violet." He pressed something into her hand. "This is for you."

A Key.