The Black Widow

Formerly an agent for the Soviet Union, Natalia (Natasha) Alianovna Romanov serves as a freelance agent, often for SHIELD, as the Black Widow. After defecting, she offered her services to the organization and Fury accepted, granting her an extended visa and using his influence to bestow upon her special amnesty for previous espionage crimes. Like Logan, the Widow's espionage background helps her to understand Fury's motivations better then others like Captain America and Iron Man who are often put off by Fury's acts of subterfuge. 

This mutual understanding has lead to many successful pairings of Fury and the Widow, among them the prevention of the attempted assassination of President Carter and more recently going up against cyber-enhanced terrorists known as the Tyrannicals. Her stint as leader of the Avengers also lead to better relations between that organization and SHIELD.

Her close relationship to SHIELD, and especially Fury has allowed her to take advantage of the agency's resources for some personal missions, including clearing the name of her mentor Ivan Petrovich from a plot by Damon Dran and helping to maintain her mantle of the Black Widow from a new upstart, Yelena Belova. She currently serves as a reserve special agent to SHIELD, answering directly to Fury.

In recent years their friendship has proven invaluable as both have gone on the run as their respective worlds have grown even more dangerous. Fury called on Widow to help during his 'Secret War' in Latveria, an act for which Fury paid a heavy price. Prior to his having to leave SHIELD, Fury alerted Widow that she was being called in and as director of SHIELD gave as much help as he could (including enlisting Daredevil) to help her evade her enemies. With the new world order following events such as the superhero civil war and the ascent of Norman Osborne and the HAMMER organization, both Fury and Widow are spies left out in the cold running doing what best they can to restore order to the world while keeping low profiles..

The second Black Widow, Yelena Belova, following her dealings with the original Widow and Nick Fury in the events of Black Widow: Breakdowns apparently also joined SHIELD as a freelance agent (a level 5 agent) as per her appearances in The New Avengers #5-6. Later she began working for the Thunderbolts during Norman Osborne's tenure as head of the SHIELD replacement agency HAMMMER..