She's Natalia when he finds her, leaning against the side of a crumbling building in some frozen little village. She's sure that's what it says on her file, Natalia Romanova, despite no one calling her that for years. It sounds weird in his mouth, flat and rough.
She Americanizes and becomes Romanoff when they land in the states. It's not hard, leaving Romanova behind. It feels merely like shedding another identity, like she's done so many times before.
She makes them call her Natasha, the ones who are brave enough to use her first name anyway, which is really just him. He tells her he doesn't understand how you get that from Natalia, but smiles a little when he says it, and she finds she much prefers the way he pronounces Natasha.
The nicknames develop over time. She becomes Nat and Tasha and even Tash on occasions. She knows she should mind, should hate it, but for some reason she doesn't.
She's Nat almost all the time; when they're talking or teasing or flirting. When she kicks his ass during sparring. Whenever, wherever. She likes it, honestly. It's the first real nickname she's ever had, aside from Natasha, he's the only one who dares to call her that. She kind of likes that too, in a weird sort of way.
She's Tasha when he's worried about her. He'll never admit to it, but it's obvious. When she goes down during the fight in Madrid all she can hear over the comms is "Tasha Tasha Tasha". It's a through and through and she's fine soon after, but for weeks that's all he'll call her.
She's Tash when they sleep together, and he rolls over in the middle of the night and buries his head in her neck and mumbles against her skin. She likes that too, in a different way, a way that she doesn't ever want to think too deeply about.
She becomes Natasha when he's mad, at her or at the mission or Coulson or whatever. He snaps and whines and complains and she rolls her eyes, because he has a tendency to overreact and sometimes she doesn't want to bother. And then he stretches out her name and whines like a child and she can't help but laugh.
She has had a thousand aliases, taken on a thousand names. She knows how to wear them, how to slip out of an old one and slip into something else, using it as a second skin, as armor. Her nicknames are different, though. She finds she can't slip out of one and into the other, despite their different uses. They're all different parts of a whole; different parts of her.