There have been many Dopplegangers over the years, from the romantic spy of the 60's who vexed Cold War Allies by seducing their agents to the Communists to the shapeshifting hero of the 80's to the face-stealing psychopath of the 90's to today's throwback who hails from the hidden city of Dopploppolis and can duplicate any superbeing that they see.
But the one who was a legend was WWII's master of disguise, a former actor who joined the war effort and used his uncanny skills to infiltrate first the French, then the British, and finally worked his way to America, where he lead a fifth column of saboteurs against the industrial cities of the US. He was finally killed by Detroit's namesake hero, Detroit Steel, when he leapt at the distracted hero, sailed over his head, and fell into the gears of the assembly line that he was trying to blow up.
Upon his mysterious return, he acclimated quickly to the modern world and now seeks to complete his final mission by bringing America down. He wears hats of both leftists and rightists, sowing dissent by enflaming each side against the other. He can be seen whispering in the ears of politicians, business leaders, and protest leader, then anonymously posts on the Internet, dropping dank memes and false information. He knows that there is no greater weapon to turn against the nation than itself and is gleefully watching it tear itself apart.
As for Detroit Steel?
Doppleganger sits in a café in downtown Detroit, watching the city rot around him, content to not so much as lift a finger as the city slowly crumbles. Should another hero take up the mantle? They'll find Doppleganger ready to balance an old debt.