He sees her sometimes—a flash of blonde hair disappearing around the corner of the TARDIS. Even though he knows that it isn't her, that it can't be her, he chases it down every time.
And every time, she vanishes in front of him, just like she'd done the first time, when she disappeared in Pete's arms.
He should know better. Mirages are never real. But he chases after her every time, because what if this is the time that she is? He runs, he calls her name, hoping to see her cheeky smile.
Instead, she vanishes into thin air.
She sees him sometimes—the tailcoats of a brown overcoat disappearing around a corner in front of her. Even though she knows it isn't him, that it can't be him, she chases it down every time.
It's always another man, dressed like him, with hair like his. It's never him.
She should know better. That day on the beach, before he vanished, he told her it was impossible. Two worlds would collapse. Still, she chases after him every time, because he'd been wrong before, and what if he found a way through?
But every time, her hope is a mirage.