Iron Man 2 (2012), The Avengers (2012)
The Black Widow (also known as Natalie Rushman, Natasha Romanoff, Natalia Alianovna Romanova) is an under cover spy for S.H.I.E.L.D., given the Soviet equivalent of the American Super Solider Serum (used on Captain America) with the uncanny ability of psychological manipulation, quick mental processing, situational awareness and rapid response reflexes. A world class master of various martial arts, gymnastics and weaponry.
Captain America: The Winter Soldier (2014), Avengers: Age of Ultron (2015), Captain America: Civil War (2016)
Emily Blunt was originally scouted for the part but passed due to scheduling conflicts.