U.S. Agent aka John Walker first appeared in Captain America #323. John Walker is a former military pilot that received superhuman abilities after agreeing to treatments from the scientist known as the Power Broker.
First appearing as a villian known as the American Patriot, John Walker was selected to be Captain America (Captain America #333) when Steve Rogers quit due to pressure from the Commission on Superhuman Activities.
John Walker’s tenure as Captain America was filled with constant moral strife and became short-lived when his secret identity was exposed to the world. As a result of the exposure, John Walker’s parents were killed by a terrorist group known as the “Watch dogs.”
When Steve Rogers decided against resuming the mantle of Captain America, it was John Walker that convinced him to change his mind. In order to give John Walker back a secret identity he was killed at the public press conference announcing his resignation. His death was faked by the governments so that he could assume the identity of the US Agent.
To ensure no one identifies Walker as the US Agent, he is hypnotized into believing his parents are still alive, and assumes a new cover identity of “Jack Daniels.” He undergoes speech therapy and works on erasing old mannerisms to hide the fact that he was the man the public had recently seen “assassinated.”
The US Agent has been a member of both the East and West coast Avengers, Force Works, Invaders, Omega Flight, S.T.A.R.S. (Superhuman Tactical Activities Response Squad) and the Dark Avengers. He eventually is fired by the government and becomes a private security consultant. But that is NOT the end to his story….