I feel like this story has been done a lot, but I wanted to throw my version onto the pile, because they'd be such a great couple.
. . . . . .
They're yin and yang, two sides to the same coin. He's a sniper, she excels at hand-to-hand combat. He observes situations from a distance, she gets up-close and personal to get what she needs. Their skills and personalities perfectly complement each other; that's why they work so well as a team, and he wouldn't want her any other way . . . usually.
But there is one way in which he's dissatisfied with how her personality contrasts with his own: he believes in love—loves her, if the truth be known—and she doesn't.
Or at least she won't let herself, and in some ways that's just as bad. He knows she cares for him as much as it's possible for her to care for anyone; she's been closer to him than his own skin for years now, with the sort of desperate bond that comes with relying almost entirely on one person for safety and help and companionship. They fight together as partners, each covering the other's back, and when they return to the base they stay stuck together like glue. They're both loners, as is inevitable given what they do, and while they're on friendly terms with most of their SHIELD associates—him more than her—it's a careful, guarded friendship that comes with being an assassin and sticking to the shadows. It's only with each other that they open up, speak honestly, dare to trust. He knows from the others how she reacted when she heard that he'd been compromised by Loki; he awoke from that enthrallment to find her sitting by his bed; he heard the worry in her voice; he knows that she'd do anything to get him back.
But love? Never, not for her. He tried to speak of it once a few years ago—of the fact that it's more than survival that keeps him by her side, that he's happier when she's near him—and she cut him off quickly, with an expression that told him not to bring it up again. So he has to be content with their desperate reliance on each other, with never being allowed to touch her like that, with the fact that they will never lie in each other's arms and speak the quiet, affectionate things allowed to those who have finally ended up where they wanted to be.
Except the one time that they did, and he carries that memory with him like an amulet to ward off despair and defeat. When the job gets to be too much, when she bids him a curt good night after a heartfelt conversation, he remembers Budapest.
But apparently this is another way in which they view the world differently, because when they're caught in the streets of New York City, embroiled in a deadly firefight with alien invaders, she turns to him with a grin. "This is just like Budapest," she says, and all he can do for a moment is stare. So this is what she thinks of, when she thinks of that mission? And for a moment, he's back there.
They're holed up in a safe house, or at least what remains of one; the scattered debris and dripping water don't exactly inspire feelings of safety. But it's unknown to their enemies and they're sheltered from the weather, so it's better than nothing. And they've got more on their mind than their accommodations anyway; their target had been tipped off and was waiting for them in the street with an armed force, and the firefight that followed was worse than anything they'd ever seen. Through some miracle they still stole back the diplomatic pouch, but in the process he was shot clean through the right shoulder. It was a lucky thing it had started raining, or what remained of their target's security team would have easily followed the blood trail he was leaving. And in fact he was so fearful of that happening that he told her to go on, make the drop, he'd catch up with her later. She hadn't even considered the suggestion, just taken his hand to tug him along as his pace started to flag.
And once in the relative shelter of the ruined house, she manages to find part of a first aid kit and patches him up as best she could; it'd be much better to get him to a real medical team, but the SHIELD agent on the other end of their radio says they couldn't be retrieved until the morning. She asks about local hospitals and is told they should all be considered hostile, and he can see from the look on her face that she's considering taking him in anyway. But they both know wounds well enough to know that if this one didn't kill him right off, there's a decent chance he'll survive until morning, and in their line of work, a decent chance is usually as much as you can expect.
So she makes him as comfortable and warm as she can then goes out to do a perimeter check. He would have helped but he's feeling shaky and woozy and he's not sure he could have gotten to his feet. "SHIELD's monitoring the location," he points out when she returns, wet from the spots where the roof didn't keep out the rain. "They'll let us know if anyone approaches."
"I know," she says, sounding frustrated, which is unusual for her. "It's just—"
"It's just?" he prompts after a moment.
"It just, why couldn't they retrieve us now? If something happens, you can't run."
"You can," he points out. "You'll just have to get the package to safety."
And she fixes him with her most serious stare. "And leave you to die?"
He shrugs, which is a bad idea because it moves his injured shoulder. He tries to hide the wince, but she knows him better than that and kneels at his side to check the bandage. "It'd be your duty to leave me here to die," he points out as she carefully smooths down the tape holding the gauze in place.
"And you think I'd obey that order?" she asks, standing and moving to the other side of the room to watch the rain fall through a glassless window frame.
He wishes he could see her face. "I don't know," he says honestly.
And she turns back to him, and for once he can read all the concern and fear in her eyes. "I don't know either. And I don't want to find out what I'd choose in that situation."
He doesn't want to know either. He doesn't say what he's thinking—that he'd never leave her behind, if their roles were switched—and she doesn't ask. After a moment, she returns to sit by his side, her delicate fingers playing idly with the sleeve on his good arm. "When you were hit . . ." He holds his breath as he waits for her to continue, thinking that surely his heart pounding this much can't be good for the bleeding wound on his shoulder, but he needn't have worried. "I'm glad it wasn't worse," she finishes lamely, and he knows it's as much as he's going to get from her.
"You should lie down," he says, resigned. "It's been a long day, and this is the only dry spot in the room." And normally on a mission like this, they'd sleep side by side but not touching, but this time—he doesn't know if it's the cold or if she's still shaken—to his surprise she curls up at his side, putting her head on his good shoulder and moving his arm so it wraps around her, and she places her hand on his chest and plays idly with one of the pockets on his uniform, and he might be slowly bleeding to death on a dirty carpet in a wet room with only a jacket for warmth, but it's still the best sleeping arrangement he could possibly imagine.
"If Fury sees you like this he'll have your head," he warns her, and he can feel her laugh quietly against him.
They lay there quietly and he's almost asleep when she speaks again. "I wouldn't leave you here to die," she says unexpectedly. "Even if it went against protocol. Even if the mission failed."
He tightens his grip on her. "Me neither."
And in the morning they awake at the same moment, and her eyes quickly go from the bandage to his face, and in those eyes he sees her relief that he has survived the night. The next moment she's on her feet radioing for their transportation, and they return to base and never speak of that night again. But he remembers it, thinks of it every day, and every time he does he wonders if she thinks about it as well.
So when she says, in the midst of the swirl of alien invaders, that the firefight is just like Budapest, he realizes she does remember that mission. But instead of remembering it as the only time they touched each other as though they cared, she remembers it as a thrilling firefight. And for not the first time, he wishes that in this, they were not so different, that the notion of two assassins finding love was not so thoroughly impossible to her.
And it's with a touch of resignation that he responds. "You and I remember Budapest very differently."
. . . . . .