His Impossible Girl.
His Indispensable Girl hit a bit closer to the mark, but didn't have quite the same ring.
His Clara.
She'd saved him, so many times, and now she'd done it again. A young, human, girl, with the cheek to call out the almighty Time Lords of Gallifrey and demand they do right by their wayward son. He literally wouldn't still be here, if not for her. She astonished and humbled him.
He hoped, rather desperately, that this all wasn't too much for her. True, Clara understood a good deal more about his people than others that he'd traveled with, but still…she'd seen him age, then become young again, then regenerate, all in the space of a few hours of her own personal time line. It was a lot for anyone to take in. Hell, it was a lot for him to take in. He wanted - no, needed - her to be all right with this. He couldn't bear the thought of starting off on a new lifetime of adventures without her hand in his.
She was his Clara.
That was all he really needed to know, wasn't it?