Black Widow

Origin
As an victim of the Red Room, Natasha Romanoff is an excellent assassin, and recruits Bruce Banner in the Avengers to begin the fight against Loki. In Infinity war, her hair color changes from her signature red to blonde. A theory of why this is, is because she is a criminal for helping Steve Rodgers in Civil War. Which is also why Captain America Looks different.

Movies
Iron Man 2 (2010)

Avengers (2012)

Avengers Age Of Ultron (2015)

Avengers Infinity war (2018)

Avengers 4 (2019)