from Avengers: Endgame

“Even if there’s a small chance that we can undo this, I mean, we owe it to everyone who is not in this room, to try.” 

In the Marvel Universe, the comic universe that is, Black Widow is one of S.H.E.I.L.D.’s best assassins but she was once on the bad guys’ side.

Born in the Soviet Union, Romanoff was orphaned and rescued during an attack in Stalingrad.

She was recruited by the KGB to join the Soviet army and was in World War I. Then she was recruited into the Black Widow program.

BLACK WIDOW'S  HISTORY

Following World War II, Natasha progressed into the Black Widow Program, where young girls like her were conditioned to become sleeper agents. Trained at a facility called the Red Room, Natasha was also enhanced with the Soviets’ version of the Super-Soldier serum, gifting her with peak human strength and stamina, as well as resistance to disease and slowed-down aging.