Disclaimer: I own nothing.
A/N: SPOILERS for the Avengers movie. You have been warned.
There is a list of things they don't talk about. The list is unwritten, but they both know exactly what's on it. Almost everything on the list was discussed between them once, on long missions, dark rooftops, sleepless nights…but once was enough. There are a lot of things on it: information about past lives, fears, doubts, all things they can know about each other but nothing they want to openly discuss.
Clint's never been bothered by the list. For him, it's nice to have someone who knows everything and isn't trying to use that information against him. He knows Natasha feels the same way. They're partners, and somewhere in the middle of close calls, late nights, and everything in between, two broken spies learned to trust each other.
He broke that trust. Loki was in his head, controlling him even as the tiny part of his mind that was still Clint Barton and not Hawkeye screamed at him to stop. He told Loki everything, about SHIELD, his life, and he told him everything about Natasha. He didn't know whether or not she knew, didn't know if she had even spoken to Loki in the time he was held at SHIELD. They hadn't talked about that yet. But he saw her face when she whispered that she'd been compromised and there are very few things in the world that could ever bring the Black Widow to that state. Clint felt violated, betrayed by his own body and mind.
"How many agents did I…"
Natasha wouldn't tell him what the casualties were. Instead she gave him a look and filed the number away on the list. He knew she didn't blame him, not for going rogue, not for anything. She trusted him whether or not he was comfortable trusting himself. It was just how they worked.
They have another list as well; of things they only talk about when they're alone and only in specific circumstances. That list doesn't hurt so much to think about. It has the first time he kissed her and the first time she reciprocated, the first time they made love and every time after, even an ill-advised marriage proposal that she wrote off as a joke and he doesn't know how to tell her he meant it. That list is special and only for them. That list is the things that will never appear in a SHIELD file and have nothing to do with security clearance. It's just for them.
Some people go on dates or make vows and wear rings to show the public they belong to someone else. They have lists. Hawkeye and Black Widow. Clint Barton and Natasha Romanoff. In whatever title they've adopted for the day, she is his.