Natalia Alianovna "Natasha" Romanoff, better known as Black Widow, is one of the best spies and assassins in the world. Originally an agent of the Soviet agency for foreign intelligence, the KGB; she later became a member of S.H.I.E.L.D., the international counter-intelligence agency. Having extensive mastery in the martial arts and armed with her Widow's Bite, Black Widow was one of S.H.I.E.L.D.'s most talented agents. When Loki declared war on Earth, Black Widow joined the Avengers and helped to defend New York City.
After the Avengers defeated Loki, she continued her work with S.H.I.E.L.D., this time working with fellow agent Captain America. Following the HYDRA Uprising, which resulted in all of her morally dubious history being revealed to the world, she dropped off the grid to begin rebuilding her cover. She later rejoined the Avengers, working to bring down various HYDRA cells across the world and the rogue artificial intelligence Ultron. Unlike most members of the original Avengers, Black Widow remained a member of the second incarnation of the team after the Ultron Offensive.
As the governments of the world demanded the Avengers to sign the Sokovia Accords, she agreed to their terms. She later joined Iron Man in a fight against Captain America because of his disagreements with the Accords and his criminal activities with the Winter Soldier. As the two Avengers' factions fought against each other, she betrayed Iron Man's team in order to assist Captain America to find the instigator of the teammates' fight. As a result, Black Widow had to escape once again from the government for aiding her old friend.