Black Widow

Natasha Romanoff was manufactured, not raised: taken as a girl into the Red Room, trained as a ballerina and a killer until the two were the same discipline, and graduated into the most effective assassin of her generation. The ledger from those years never balanced. She spent the rest of her life trying anyway. An agent named Barton was sent to kill her and made a different call, and the weapon became a shield: S.H.I.E.L.D.'s best operative, the Avenger with no serum and no suit of armor who stood on the front line beside gods anyway. The debt came due on Vormir, and she paid it the way she always worked, first through the door so no one else had to be. The ledger is clean.