U.S. Agent, A.K.A. John Walker is a sometimes-rival, sometimes ally of Captain America. He began his career as Cap's enemy Superpatriot, until the U.S. Commission on Superhero Activities stripped Steve Rogers of the Captain America name, costume and shield, and gave them to Walker, who was trained by Taskmaster to throw the shield like Cap. His identity went public when his former partners, Left Winger and Right Winger, revealed it on TV.
Walker as Captain America suffered a nervous breakdown when his parents were killed by The Watchdogs, so he killed most of the Watchdogs and the few survivors got heavily injured. From then on, he became increasingly unstable. He went after Left Winger and Right Winger, blaming them for his parents' deaths, and after defeating them, left them tied to a petrol tanker about to explode. They barely survived, albeit with grave burnings.
When Flag-Smasher found his organization ULTIMATUM was funded by the Red Skull, he threathened an U.S. arctic base to lure Captain America there and ask his help, but after noticing this Captain's disregard for human life, he captured the "imposter" and told Walker's partner Battlestar to bring the real Captain America. Battlestar returned with Rogers (now as the Captain) and D-Man, and while Battlestar freed Walker, Rogers and Flag-Smasher destroyed the Red Skull's doomsday weapon.
Later, the Red Skull called Walker to the hospital to come to the Smith building, where he used his cloned body to make Walker believe he was Steve Rogers, and left his henchmen to get rid of Walker. When Walker defeated them (it's unclear if he killed them), he found the real Rogers and, believing him the leader of his attackers, fought him. Rogers defeated Walker, who realized the truth when the Red Skull came to gloat in front of Rogers. Walker saved Rogers from being shot with Red Dust, which backfired on the Skull instead. After the Red Skull escaped, the CSA asked Steve Rogers to return as Captain America, but he refused until Walker himself convinced him that he deserved to be Captain America again.
Taking the uniform which Steve Rogers had used as The Captain, the CSA redeployed Walker as U.S. Agent, and forced the Avengers to add him to their roster. U.S. Agent still helped in some of Cap's solo adventures, like when he and the Falcon faced sme ULTIMATUM agents to find the lost D-Man.
When the West Coast branch of the Avengers was closed, U.S. Agent eschewed his "Captain" uniform as he didn't want to be known as a Captain America wannabe. When Iron Man created the team Force Works, he made a new outfit for U.S. Agent, equipped with photon shields that he can create and shoot with his gauntlets.