They're wound together in a spider's web of coincidence and chance meetings, of work passed on and friends in common: binding obligations each and every one, so that with each one uncovered their six-point star becomes more stable.
See: no one knows how or where Dr Abraham Erskine began or even found the inspiration for his research. He died, after all, without telling even Howard, and Steve went to Europe in time to rescue a boyhood friend who showed every sign of responding just as well to the serum treatment as Steve himself had, and certainly far better than Schmidt. Eventually that research falls into the hands of the Soviets, who apply it to a red-headed girl with dead green eyes and sit back, curious, to watch its effects, which are almost unnoticeable.
Ten years later that girl stands across a gym from one of the other ten successful test subjects they have for the serum and thinks to herself, I can take him.
She can't. Not yet.
(At least, not in the way she means just then. She gets him to surrender something else entirely, but that's as much loss as victory because he does the same to her.)
Eleven successful test subjects in total. One, if they're going to give credit where it's due, isn't even theirs. That leaves ten.
That leaves thirty-five more, mostly adolescents when they were given the dose, dead. Their corpses are burned in a fire pit, their ashes scattered, the records of their mutations, their slow agony, their eventual madness, the way they would claw at their own bodies with their fingernails as if to peel the skin off their muscles that was no longer their own, destroyed along with their bones. No one ever finds out what happens to them; although the girl with the dead green eyes heard both the screaming and the rumours. Each terrified her in the same measure.
Fast forwards forty years. Steve's friend - Erskine's research partner - Howard has a son, who as yet is a boy, careless and carefree and an utter, utter mess of contradictory impulses and subtly-nutured resentments and confusions. While he spends his days dancing on the brink of ruination (laughing, all the while, the exhilarated, gleeful laughter of a child playing on a swing - look, look how far I can go!), an American spy brings the news that the Soviets had found Erskine's - well, HYDRA's - research after the Second World War. He could give them nothing more than a rumour of success. There was, because he had not yet met the girl with the dead green eyes, no rumour of failures. It was his first mission, and he did it well; it gave him enough clout to laugh off the sniper rifle they offered him for his next mission, and pick up a bow instead.
And the girl with the dead green eyes watches the collapse of everything she was told to believe in, and two young scientists in an American university are given their first look at something the military general calls gamma radiation.
One day the girl with the dead green eyes counts decades on her fingers and admits to herself that she is never going to age the way other people do. She might, she supposes, drop dead one day of natural causes; more likely is a knife across her throat, a bullet in her head. The prospect doesn't trouble her very much. She should, she thinks, have left far sooner, around the time she discovered what they had done to the man she - well. That could have bought them twenty years of freedom.
Of course, it probably would just have bought them a rather messy, painful death. She prefers to dish those out than to take them and does so, for a few years, for money. That's degrading in its own way, but it's freedom too: she gets to turn down orders now.
Along the way, four of the other nine successful test subjects die (quickly quietly). The girl with the dead green eyes is not fond of the notion that there are others in the world who might tell her enemies what she is, exactly. She does not seek them out, but if they're there, and pitting themselves against her anyway...
Two others have been dead for years on missions. Where the other three are hardly matters, as long as they don't cross her.
(Annika Reiter escapes the scene of her mother's murder because she has dead green eyes and thick red curls and the gun pointing at her forehead wavers and another assassin sees it.)
She has an arrow wound in her shoulder while Howard's son hires the only person he'll fully trust for nearly a decade to be his PA and the American spy with the bow and arrow says, I have a proposal for you. She likes the sound of that. She'd rather die than be a test subject, but (somewhat to her own surprise) she finds she'd rather not die at all.
They think she is the third woman to carry the codename she is known by across all the Eastern World and wide swathes of Africa. She shrugs, and does not disillusion them. Nor does she mention the rumours.
The two young scientists are growing ever cleverer. If the girl with the dead green eyes had known about them, she might have been less reticent.
On the other hand, she might have just wandered into the Pentagon one day and spiked Thunderbolt Ross' morning coffee with something that would smell of almonds. She's never really been predictable, but she does have a healthy appreciation for the notion that the shortest distance between two points is a straight bloody line.
She has a healthy appreciation for the archer as well: another choice she got to make. They stumble, together, towards something like a sense of morality. Embarrassing weight to have to drag around. Far more painful, too, than the other option.
(They asked her if she wanted anything and she shrugged and said something to read and thought of a newspaper but they brought her books instead, battered old paperbacks of fantasy and science fiction novels, so that somewhere between Dros Delnoch and the Tombs of Atuan she began to smile, and then nearly to cry, and knew that from now on she would always, always hesitate, like a fault line in her mind.)
Then, one day, Howard's son is kidnapped, and it's as if someone has pulled a string and a knot comes unravelled, twists and turns smoothing easily out into a simple straight line: Erskine and Howard made Steve and Schmidt made Bucky and the Soviets made Natasha, all from the same stuff; Tony is Howard's heir, the Cube belongs, rightfully, with Thor's father, Bruce is as formidable a scientist as Erskine himself yet a victim of the same mistakes the Soviets made and Clint is Natasha's because she made it so and so did he and now they're standing on a battlefield and Natasha says how do we do this and knows the answer before she's asked the question because there's only one way they can do this right here, right now and maybe there only ever was, for all of it.
I don't see how that's a party, she says, lying through her teeth.