Black Widow: 8 Characters That Should Appear

Barring some major revelation, as far as the MCU seems to be concerned, Nat and the Winter Soldier first crossed paths five years before the events of Captain America: The Winter Soldier. As she tells Cap, she had been escorting a nuclear engineer out of Iran only for the Winter Soldier to shoot out her tires, cause their vehicle to crash, and eventually kill the engineer by putting a bullet straight through her.

Up until then, as Nat told it, he was only a myth, and it'd be somewhat odd for the MCU to suddenly retcon that to say that the two characters had actually known each other for years. Despite that, though, an easy way to tiptoe around it and honor the duo's comic history with one another would be to briefly feature him in Nat's Red Room history.

It's believable that the younger version of her wouldn't recognize him two decades later if the training was extremely brief, particularly if she didn't know he was being cryogenically frozen time and again, or he could simply be seen observing the Red Room training without ever interacting with Nat directly. Attention doesn't need to be drawn to him, but a blink-or-miss-it appearance would be a fun inclusion and highlight how Bucky was being used for more than just carrying out assassinations of his own.