In recent years, Captain America has set his shield aside for different reasons only for the mantle to be taken up by friends of his, whether it was because he was technically dead (shield went to the Winter Soldier) or because he was his actual age for once (shield went to the Falcon), the name and uniform of Captain America has been worn by other heroes.
But neither of these guys were the first to take Cap’s name and uniform. For that, you may need to go back to the story of U.S.Agent.
John Walker was a good, proud American who wanted to be a hero like his big brother, Mike, a helicopter pilot who was killed in Vietnam. After his own time in the armed forces, Walker got wind of a guy named the Power Broker who was able to give out superpowers. He got himself some superstrength and set out to do, well, something.
Originally, he took the code-name the “Super-Patriot” and did what any patriotic fellow would do and attacked Captain America.
Though Walker would eventually sign up with the government to temporarily replace Steve Rogers as Captain America during one of those periods when Steve stepped away from his usual position as the most stand-up guy in the Marvel Universe, it’s hard to say whether or not Walker and Rogers get along very well. Though the politics of both men haven’t exactly been revealed for obvious reasons, it is probably safe to assume that wherever Steve Rogers falls on the political spectrum, John Walker is somewhere to his right. Rogers is often depicted as holding the lofty ideas of his nation close to his heart; Walker doesn’t seem to hold those values in the same regard and prefers a more pragmatic approach. Walker does love his country, but he’s also more willing to get his hands dirty and has been known to be depicted as a somewhat politically conservative fellow when compared to other heroes around him. Heck, in a somewhat recent issue of Might Avengers when Walker was a member of that team, a giant Inhuman was attacking China with a version of the Terrigen Mists that turned regular humans into neanderthal type brutes. Walker was among the first Avengers on scene to be infected, but his teammates Quicksilver managed to get him fighting on the heroic side again by simply pointing at the giant and telling the nearly mindless Walker that if the giant wasn’t stopped, the communists would win. And that worked.
If there was a difference between the two, it was Walker had a lot less compunction about using lethal force. He came pretty close to a breakdown, especially when some former teammates revealed his real name on national television and got Walker’s parents killed. His time as Cap didn’t last, naturally, and Steve Rogers would take back the uniform and shield.
Walker soon re-emerged as the U.S.Agent, wearing a variation of the Cap costume with a new Vibranium shield he probably got at a Wakandan garage sale. He joined the West Coast Avengers and has been bouncing around ever since. Again, it seems hard to tell if other heroes, particularly Steve Rogers, even like John Walker. Walker has often been depicted as a generally honorable guy, but also one who seems to enjoy being on the front lines and taking care of business, and sometimes he says or does things that rub people the wrong way. As the character was created in 1986 by the late Mark Gruenwald, it is fairly probable Walker was created as a reaction to the nation under then-president Ronald Reagan as a contrast to Rogers coming from the era of the New Deal and FDR. In fact, Gruenwald originally conceived of Walker as a patriotic villain to Cap’s hero.
And given the number of teams Walker has been on, I personally wonder if a number of Marvel creators really know what to do with him. He’s been part of the West Coast Avengers, Force Works, the Dark Avengers, and even joined a Canadian team at one point in the form of Omega Flight. And sometimes, that means ridiculous new costumes.
The Dark Avengers stint seems to be his most recent. Walker was there as someone who was mostly keeping an eye on dark variations of known Avengers, and he did it after the Siege storyline literally cost him an arm and a leg. In the meantime, anytime a discount Captain America is needed, U.S.Agent will be there.