Re: MCU-ize Marvel Characters
WINTER SOLDIER: When Bucky Barnes fell into that frozen river, he did not die in the traditional sense. During his captivity in Italy at the hands of HYDRA, he had received a experimental variant of the super-soldier serum created by the rogue Nazi organization, on that, while not as powerful as one that had been used on Rogers and Schmidt, would be able to allow for minimal side effects. However, for most of the captives experimented upon, there was a side-effect: death. Only Barnes survived, not even knowing what had happened to him.
So, whether by luck or as a side-effect of those transfusions, he survived, washed out to the sea, where he was discovered by Russians, who were rather curious about what was going on in the war between the Western Allies and HYDRA. Unable to use Barnes for intelligence (he had suffered severe brain damage and memory loss from his injuries) or science (the experimentation by HYDRA was useless without further context and documentation), they decided to use Barnes as a Winter Soldier... upgrading him with rudimentary cybernetics... and then tricking him into doing the Soviet Union's doing...
Marvel. DC. Indies. Who cares? Can't we all get along?