you were a soldier with Steve and Bucky in World War two before they became super soldiers. On one mission you went missing and were presumed dead. Little did anyone know you had actually been kidnapped by Hydra and turned into a brainwashed, super soldier, assassin like Bucky. In the present time (when all the Avengers are alive) you have still not been found out about and are still brainwashed, you’re on a bridge top waiting to attack Steve and Bucky for a mission you were sent on by Hydra.