Spy

A spy is a person skilled in espionage, watching and examine others, and stealing information. The term is sometimes used as a verb to mean to watch someone without their knowledge.

History
S.H.I.E.L.D. is a large organization essentially comprised of spies.

Chameleon, Mystique, and Morph have worked as a spy for various people, mostly due to their ability to appear as anyone.

Lewald spied on Herbert Landon for Kingpin.

Hobgoblin had a spy on Kingpin.

In the Comics
Those who have been spies in the comics but not specified as such in the Marvel Animated Universe include Wolverine, Abomination, Betty Ross, and Hawkeye.