Sometimes, when he's sure she can't see, the Doctor watches Donna.
Donna lands on top of him, dressed in ten layers of white tulle and satin. They both pant for breath, from the chase, from fear, but mostly from having the wind knocked from their lungs.
She isn't some slip of a thing, and that is new. She is older, spread with age, her hips wide, her body thickening. She is like a pillar, fixed rock solid, something steady and immovable, though the Doctor, of all people, knows that nothing is truly fixed. His earlier selves might have found her intolerably slow, accused her of old age; he wonders if he is growing old, that he can find such comfort in her stolidity.
Donna laughs, Donna asks questions, Donna faces down hostile aliens with a smirk and a snarl. Donna tilts her head with a knowing smile, with exactly the necklace he needs dangling golden from her fingertips.
He's done this often, with all his many companions, for various reasons. He remembers when he was young, when he watched them in the spirit of scientific observation, trying to quantify the many attributes that made humans human: two parts courage; two parts cowardice; equal measures of compassion, aggression; sheer, bloody-minded stubbornness. Variation among subjects, of course. Occasionally, they're even intelligent.
Donna strokes the fabric of her new toga with wide-eyed wonder. Her hosts were anxious that their noble guest be comfortable, and she was anxious to reciprocate. When in Rome. She wears it well.
He can't quite remember when he began to watch them for other reasons, new reasons, but he remembers when he first realized it: when Jamie came along, all fire, determination, tenacity, and that ridiculous kilt he'd wear in all weather. Suddenly the Doctor had a best friend, the best he'd had in a long, long time. And he remembers the surprise and wonder of the realization, that he could befriend a small, short-lived ape from an insignificant planet. He would watch Jamie, when he could, trying to determine just how, and why, it had occurred.
Donna whirls around in the snow of a planet that hasn't seen spring in a thousand years. She's almost dancing, to a tune that is nearly transcendent with joy. The sun shines bright, glinting off the ice and the air, but Donna is brighter still.
He watches Donna now to figure out how it can have occurred again. Again, after all the others, the friends that come and go, whirling into his life deliberately or accidentally, leaving again with a scream, a bang, a whimper. She would not leave him quietly, the Doctor thinks. It's not in her nature to do things quietly; even when she's not speaking, she can fill a room with her presence, an impression of bright red hair and a brassy voice. She is fierce, this human, and perhaps that's part of it. He's always liked the fierce ones: little Sarah Jane, proud Leela, and Rose.... They are so different from his own people, who gave up ferocity long ago, and he has always reveled in the difference: before, to help remind him of his people's failures, and now, well, to help him forget.
Donna smirks at him, at his babbling, unnecessary goodbye. He'd assumed she was leaving and she'd allowed it, perhaps as a punishment for doubting her, but more probably as an abject lesson; for all his genius, there's so much he doesn't know about humans; more to the point, there's so much he doesn't know about her.
They grate on each other's nerves sometimes. He has, for so many long years, traveled with the young, the new-minted humans, not yet grown into their own minds. What's amazing, and new, is that he is not afraid for her, not like he was for Rose, or Martha. Donna is too old; she will not become a soldier, or an adrenaline junkie, or anything but what she already is. He can show her things, refine her, perhaps, but he can't arrest her development, make her worse, or less. She won't let him. And he finds that when he's not afraid for her, he can sometimes fear for himself; fear, self preservation, survival: they're healthy urges, even when he ignores them. He had forgot.
"Her name's Martha," Donna says, utterly furious. "And she's not collateral damage, not for anyone. Have you got that, G.I. Jane?" Later, after, in a quixotic fit, he'll tell her gratefully that 'Donna' means 'lady' in Spanish. She'll tell him that she already knew.
She does like to give people nicknames. It makes him smile, even when he's on the receiving end. 'Spaceboy.' 'Martian.' 'Sunshine.' He likes that one. Perhaps, one day, he may use it himself. She gives everyone names, and this is so human. They have knack for it, humans do. 'Doctor.' 'John Smith.' Short, to the point. One pleasingly accurate, one usefully bland. Names with power, as so very many cultures believe. He likes that she knows that neither is his real name, the one given him by his own people. He likes that she would use it if he told her, as a talisman, something that proves him a person. He likes that she doesn't ask, and thinks him a person anyway.
Donna grabs him, and he's dying, choking on poison and protein and salt, waiting for some big shock that he thinks may not come, because he's so old, and so little surprises him. But she grabs him and kisses him, and yes, it's a shock, it's utterly ridiculous, really, he still tastes like anchovies, and it's enough. The poison spews from his mouth, and she watches open-mouthed, astonished at her handiwork.
She's not in love with him. It's been a while since that happened. His companions, so young, most of them, desperate for someone to love, and he flatters himself that they could have worse taste. And it's heady, being loved by people so very bright and brief. Heady, addictive, and so very hard when it ends, as it always must. She's not in love with him, and quite right, too. She fell in love with coffee and nice jokes, mortgages and marriage. They broke her heart. And then she came to him. It's a relief to know that, with her, there will be no illusion of romance, of settling down. They're both out to escape, partners in crime. Occasionally, he still wonders how well he'll cope without the golden glow of a human's affection. And then Donna grins at him, and he knows he doesn't have to.
Donna looks him in the eye, and she doesn't tremble. She knows now, has acknowledged that her time with him will end, and considering their life, the end could be terrible. He cannot comfort her with lies, but he can offer to show her their future. She doesn't ask him to.
So many people have died in his care. He's long since lost count. Each one he hears of hurts. Each one he knew, knew and loved, is like acid on a wound. He's always known that Donna will leave, as they all do, but he hopes it will be peaceful. She'll fall in love again, find a cause, hug him, even find him a new friend. She's that kind of person. He watches her closely now, waiting for the other shoe to fall, perhaps fatally. But she doesn't seem to think on it. She turns away, focuses on other things, maybe on him. He thinks, not for the first time, that perhaps humans are lucky, living for such a short time. They have to know it will end. Perhaps Donna knows her death is a fact. He can taste his own fear at the prospect. He wonders if she tastes it too.
He staggers back to Donna on a world made of diamonds. She takes one look and opens her arms, her two, small, human arms, and he comes into them gladly. She enfolds him; he clings to her as if she is the only safe shelter in a storm.
She is sensible, this human, so very sane. She puts on a coat when she's cold, eats supper when she's hungry, makes sure he's warm enough, makes certain he's eaten. And she's the voice of sanity in so many of the strange places he finds himself. Loud, annoying, but comfortingly reasonable, even when she's wrong.
Donna coils around a mug of tea, her hair wet and curling from the bath. Donna snaps orders at a uniformed commander. So reliable, so very there; he will fill his days with Donna: Donna grinning, Donna weeping, Donna belligerent, Donna gentle. Donna glaring, her hand on his cheek a sharp reproof. Donna sleeping, so warm he can feel it across the room, so steadfast he will time his breath to hers.
When they don't agree, they butt heads, move nowhere in a clash of titanic personalities; occasionally, and this is still a novel sensation, he loses. He doesn't give in, he doesn't allow it, he actually loses. Amazingly, the world doesn't end. And when he comes away from other fights, defeated, stunned, wounded, he realizes that he has survived the experience. He can walk back to his friend and bury his head in her shoulder. Maybe he can tell her about his day, how insane and horrible it was, and she'll give him tea. And later, he will still fight, because he must, use his words and his screwdriver and his clever, clever brain, but maybe, just maybe, he doesn't always have to win. It feels a bit like freedom.