Their first kiss is in Budapest. It's harsh, punishing and damnit, don't you dare do that again.
It's a signal, a non-verbal threat that she's not allowed to go into a burning building on a hunch ever again and he's certainly not allowed to abandon his post and follow her.
They both know that Coulson was following them on the comms units, that he heard Clint shout her name, her real name, over the communications feed as she ran into the flaming foundations because he was aware of someone barking 'Barton!' into his ear as he followed after her. Damnit if he was going to let his partner go up in smoke too. She had countless forms of back up but was nowhere near prepared to hunt through a building that looked like it could collapse at any moment. So he left the gunfight behind them, abandoning the other agents to the hailstorm of bullets, and chased after her.
He finds her, a burning flash of red in between flames as she climbs over a fallen post and drops behind the wall of smoke that separates them after her. He shouts her name, choking on the thick ash that fills his lungs with each breath. She doesn't reappear immediately, she comes back an agonising thirty three seconds later using only one arm to shield her face from the flames that she pushes back through because the other arm is holding a bundled blanket. A screaming bundled blanket. A bundled blanket containing the one child that was left behind.
He gives her a furious glance and uses his arms to make up for her useless one, bringing both her and the child she insisted on saving back out into air safer to breathe. The hospital had been on fire for thirty minutes, explosions in other wings, and he wants to tell her that after breathing so much smoke for that long that the kid probably won't live, but she finds one of the medical staff and hands the bundle over and it takes all he has not to throw her against the wall and demand an explanation.
He saves it for when the fight is over, for when they retreat back to the crappy motel that has three rooms under the names of Smith, Brooker and Arimov instead of Coulson, Barton and Romanoff. Coulson disappears without a word except to inform them of the mandatory debriefing the following morning. Natasha goes to enter her room and he follows her, pushing her against the wall with one hand and slamming the door shut with another.
The fact that she doesn't break his neck within three seconds tells him everything.
"Don't you ever do that again," he snarls at her.
She says nothing but looks up at him, eyes blazing with her own anger. "Don't follow me next time," she growls back. "You're not my bodyguard and I don't need you to protect me."
He forces his lips to hers, relentless and unforgiving as he brings them into a first kiss that should have been a thousand times more tender than it was. He isn't sure what he's trying to convey as he forces his tongue into her mouth, exploring her, but he wonders when she reacts and kisses him back if perhaps he's searching for the answer she won't give him and never will. She gets the message though, she knows that if she does something that stupid again that he'll kill her himself. As partners they're responsible for each others backs, you can be damn sure that if one of them died the other would get a disciplinary instead of comfort for their loss, and he won't have her risking her life for a child that probably didn't live through the afternoon becuase of smoke inhalation.
But she's good, because no man left behind doesn't extend simply to her partner.
Because she has perfection waves of judgement calls.
Because she was once the child left behind in a burning hospital, and it wasn't the good guys that came for her.
Their last kiss is much more tender, brief, and yes, loving.
They have lost count over how many times they've brushed lips this way, casually and even somewhat friendly. It's so different from that first night he threw her against the wall and gave her the first order in the field he'd ever give her, the last as well. He'd thrown her against the wall many times since, just like she's dropped him to the ground, but every time after that had been accompanied by predatory grins and the kind of cries that only he could bring from her lips.
Their last kiss comes twenty two years after Budapest.
They don't marry, never do. Marriage is for suburbia, and they aren't there, not yet. They end up in downtown New York for what is supposed to be their last mission. Their resignations are signed and waiting on their shared bed to be handed in the second they're done with the debriefs. Cases are packed, one for each of them with all the possessions they own. A signed lease, for once with their real names, and they're ready to move in tomorrow. A small two bed house in the middle of nowhere, plently of land and trees, and perfection for their personalities. They're ready to go, just one more job first.
They're suited up and filling with the adrenaline that after so long now has started to kick in before a mission has even started.
It's a good luck charm, a brief kiss, a tiny teasing brush, a reason to come home, he tells her once. Because if they deepen a kiss before a mission it leads to something more, and Fury wouldn't stand for them being late or distracted in the field. Not again.
This time, there's nothing to come home to.
That tiny kiss is the last they'll ever have, because she does it again, she goes back to save one more life, to erase that last line of red from her ledger, and this time she doesn't count the life saved as her own.
She runs into a burning building twenty seconds too late, and a rafter comes down on top of her. He goes after her, naturally, with Coulson's voice barking down his ear again, and it's all too familiar that he expects to her to walk through the flames with that bundled blanket again, but instead he finds a motionless heap of uniform surrounded by blood that has pooled on the ground from her head. Her skull is cracked, blood in her ears, and her beautiful face is burning more by the second.
The burn that covers his entire right arm will be the permanent scar he carries in her memory, but is worth the pain of not leaving her body to perish in the burning building.
They begin and end with fire, and he recalls that first order he gave her, not to do that to him again, not to go back again, not to scare him like that again.
He recalls that she never did agree to that.