It was something they all knew innately. It was never said aloud. Never alluded to. Just a truth that every member of the team learned on his own.
It wasn't that they were always together. They were just never apart. Where one went, the other was usually close. They were uncommonly aware of where the other was at all times. Consummate partners, constantly watching the other's back. Willing at any moment to sacrifice their body for the other.
Thor saw it first.
Although there were not many female warriors on Asgard, the bond that they had was as clear to him as anything he had ever seen.
Their abilities in battle were flawless, even when separated from each other. They moved and adapted to the situation, never hesitating or second-guessing a decision. Hard skills won over time and perfected through pain.
They were as fierce as any Asgardian and every man they put down stayed down. Everything they said, every mission they referenced, spoke of deeper pain, and of lessons learned the hard way.
He watched them carefully at the Tower as well. They did not give sidelong glances or leave lingering touches as lovers did, but there was an intimacy in every exchange that spoke of a deeper connection. The Iron Man often teased them for it, but it did not bother them.
Thor could see words in their silences, and sentiments in their expressions, where others saw stoic masks and indifference.
Even when they were home they never fully relaxed. When one tensed, the other did as well.
They did not train like the others. Everything they did, they did so with their fullest effort. They did not pull their blows or soften their hits.
Thor understood that this was the highest level of trust. Of partnership. If they made a mistake in training they had to learn from the pain. A bruise would be forgotten. A broken wrist would not.
They both had a warrior's spirit, and he grieved for them.
Steve saw the connection but it took him a while to understand it.
It took him the longest to understand the depth of their bond because he couldn't reconcile his own views on women with Natasha. He certainly had some experience with feisty women, but Natasha was very different from Peggy.
There was no neat little box that he could fit Natasha's personality into. She never acted or reacted how he thought she would, even after the time they'd spent together.
But Clint always knew.
It really hit home for him after they'd returned from a mission. Although they'd won, the damage done to the area and the loss of civilian life had been catastrophic and no one had felt victorious.
Natasha, in particular, had seemed to take it hard although none of them knew why. She'd sat staring off into space at the kitchen counter, oblivious to the rest of them and their attempts to talk to her. They had all looked at each other, unsure of what to do next. But Clint strode right over to her, yelling, "Get over yourself Romanoff!"
It startled them all and Natasha whirled around, "Fuck you!" She made for the stairs and Clint laughed, his voice venomous, "That's all you're good for, right? Run away when things get too hard. Black Widow can handle anything except emotions. Isn't that right, Natalia?"
Steve had been shocked. He moved to tell Clint to back off when Natasha had thrown herself at him. Her strikes were hard and calculated, always aiming to inflict the maximum amount of damage possible and screaming at him the whole time.
But Clint didn't hit back. He blocked her attacks when he could and took her blows silently when he couldn't. Steve leapt to interfere but Thor laid a hand on his shoulder.
A moment later, Natasha broke in Clint's arms.
He held her tightly as they sunk to the ground and she collapsed into his shoulder.
From then on he stopped trying to fit Natasha into a box, stopped trying to figure her out. Stopped trying to figure them out. He accepted who they were and recognized how important they were to each other.
He mourned for their future.
Bruce had watched them carefully from the beginning.
He had a little more experience with Natasha at first and actually felt as though he understood her fairly well. And then Clint had come back and he'd been forced to reevaluate everything he thought he knew.
He'd had it harder than the others since he wasn't able to observe them in action most of the time. If he was fighting with them then he was the Other Guy and his memories were hazy at best.
Those rare times when he wasn't part of their mission were spent watching over the team obsessively. He worried for everyone's safety and sanity. Everyone except theirs.
Of course he cared, but for some reason, on a team with a god, a superhuman soldier, a man in a flying metal suit, and a failed science experiment, the two humans had always seemed untouchable to him.
They'd been injured, more often than not when the team returned it was one of the two of them who had been hurt, but it never fazed them in the way it should.
Steve acted like the solider he was; he obeyed the doctors and sat out missions when Fury told him to.
Tony whined like a child and snuck off, only to be caught within the hour and strapped to a bed.
Thor, on the very few times he'd been hurt, they'd had to sedate in order to keep him there.
But Clint and Natasha just rolled with it. It was more the mental aspect than the physical one where their injuries were concerned. They didn't process or show pain in the way they should have. They frequently patched up their own injuries and almost never stayed in the medical wing for half as long as they should.
He'd come across Clint once in the training room, his arm hanging limply at his side, the sling long since discarded, with Natasha circling him predatorily. He'd asked them why.
Clint had given him an odd little laugh, "How else will I know how to move with a broken arm in a fight if I don't practice now?"
Bruce had fled the room after Natasha landed a roundhouse kick against his shoulder but Clint's scream haunted him for days.
On a mission a few months later, Clint's arm had been dislocated but there hadn't been time to get him to safety. Bruce watched from the command center as the archer never even slowed down and adjusted to his handicap without difficulty.
He got it then. He understood their need to constantly improve. Part of it was undoubtedly their survivor's instinct and some of it was their need to prove that they belonged on the team.
But the overwhelming drive was to protect each other. He'd never understood how Natasha could fight an injured Clint with the intent to do more damage. As he watched Clint move now, he saw the glances that Natasha kept throwing his way and he realized that she'd hurt him then so he wouldn't be hurt now.
It was a twisted relationship, something that he wished he never understood, let alone witnessed every day, but he cherished their friendship, and he prepared himself everyday to lose it.
Tony had started to see it from the moment he'd met Natasha, back when she was Natalie and her hair was long.
More than anything else that had struck him that day, he remembered the absolute certainty that she had a man in her life.
She'd been cold and in control the entire time, hardly batting an eye at his obvious attempts, first at flirting, and later at provoking any kind of reaction. As he watched her more, especially when she was with Pepper, he realized that her attitude wasn't borne out of professionalism, but out of contentment.
Certainly there was a large part of her attitude that was entirely her and he didn't want to touch that. Aside from being hideously incompetent at dealing with delicate topics, there was a level of pain hidden within her that he didn't want to poke. He had enough trouble dealing with his own issues thank you very much.
When he saw her again on the Helicarrier he could guess at the mystery man's identity. She would have been acting differently if he'd been killed and there was only one agent currently alive but in a compromising and potentially dangerous situation.
Despite everyone's assumptions, he was very good at observing things and reading people. He wasn't tactful on purpose because being blunt got him more reactions.
The invasion had been stressful on everyone but when he looked back later, he realized that despite the overwhelming emotional stress that Barton had to be crushed under, the man had held himself pretty aloof.
He'd molded into the group easily enough and he could recognize the same blank face that Clint showed everyone as the one that Natasha used.
As he got to know Clint better and watched him with Natasha he realized that the two of them were soul mates. Not in that cheesy romantic Nicholas Sparks way. But their worlds revolved around the other.
Their understanding of each other was without question, so completely absolute. They saw the dark stains on their souls and forgave the other while still trying to wipe the blood off their own hands.
Tony had no illusions that the two of them had seen more- dealt with more- than he could ever begin to comprehend. That kind of work took a toll and they depended on their other half to help them out and accept them.
And in his own damaged heart, he hated himself for knowing what was to come.
So they watched, and waited. There was no doubt that the day would come eventually. Probably sooner, rather than later, with the line of work they were in.
They all knew it, secretly dreaded it, wrestled with it in the dark corners of their mind.
Because when- not if, but when- one of them died, the other would shortly follow.