Summary: It was the one promise he never expected her to keep. Post Doomsday.
"How long are you gonna stay with me?"
"Forever."
Unbeknownst to her, she'd kept that promise. There were times, especially at the start, that he wished he couldn't hear her voice quite so clearly, or remember her face so perfectly. They were painful reminders of all that he'd lost and could never get back. On the worst days, he looked forward to the time when her memory would fade, joining the ranks of all the ones who'd come before her.
But she was always the different one.
As the years wore on, he grew used to having her memory right there with him, always in the forefront of his thoughts. She was his unseen companion, a stowaway never revealed to others. He was grateful for her bright smile on the lonely days and for her tempering words of patience on the rough days. Eventually, he ceased to question the wonder of her everlasting memory and was just plain grateful.
Even through another regeneration, then two, then three, she never faded. He could hear her words, all her words, as clearly as when she'd first spoken them.
"They keep trying to split us up, but they never, ever will."
"I'm not leaving you."
On the long times in between, when he drifted alone in the universe, her memory sustained him. They ate chips in London. They danced in the TARDIS, her hand in his and her bright eyes sparkling. They held hands, fingers intertwined on a frozen beach under a frozen wave. They solved mysteries, stopped invasions, righted wrongs, saved the universe, defended the Earth. Over and over and over again. The stuff of legend.
On and on she stayed, his partner – his friend – for millennia.
Even as the centuries drew to a close, and his final regeneration was breathing its last, it was her smiling face that swam before him, reassuring him as he reached for the darkness. Her name, her beautiful name, fell like a whisper from his lips and followed him into the void.
"Rose."