Black Widow (Natasha Romanoff)

Black Widow is a S.H.I.E.L.D. agent/spy. She is also an Avenger.

Origins
Black Widow used to be a Russian spy. Then she got recruited to the Avengers along with Iron Man, Captain America,Thor and Hulk. She also worked with Hawkeye.

An Avenger
One of Black Widow's missions is when she and the Avengers "defeat" the Red Skull and his Hydra army. Later on when they bring Tony (Iron Man) to the Avengers Mansion, Hawkeye and Black Widow get mind controlled by M.O.D.O.K.

Trivia
Hulk only trusts Black Widow to kill him if he goes rogue.