"Natalie Rushman" becomes Tony's new personal assistant when Pepper becomes CEO. Fury later reveals she is S.H.I.E.L.D. agent Natasha Romanoff.
Natasha recruits Bruce Banner, frees Clint Barton from Loki's control and uses Loki's scepter to close the portal over New York.
The undercover spy from Iron Man 2 becomes one of the six founding Avengers.
Source: Wikipedia: Natasha Romanoff (Marvel Cinematic Universe)