For nearly all the time I’ve been reading comics, it was an intractable “rule” that Bucky would always stay dead. His death was integral to Captain America’s back story: he was the ghost of failure that would forever haunt the Sentinel of Liberty. To his credit, he stayed dead for 40 years, which is a far longer time in the dirt than most heroes spend in these frequent cycles of death and resurrection.
James Buchanan Barnes' bio would’ve been so simple to write up before his resurrection in ’05. He was an orphan adopted by an army camp who became their “mascot” in as much a capacity as any teenager could for a military unit. After he accidentally walked in on Steve Rogers’ tent and discovered that the unremarkable enlisted man was actually the super-soldier, this little smartass more-or-less blackmailed the hero into letting him tag along on his missions as a costumed sidekick.
That was generally how such partnerships began in the Golden Age of comics.
After several years of adventuring that seriously bordered on reckless child endangerment, Bucky met a tragic end while he and Cap were trying to stop the flight of Baron Zemo’s automated, airborne drone. While the plane's explosion sent Steve Rogers into icy waters that would freeze him for the next few decades, its fire summarily killed Bucky Barnes. The image of a falling Bucky desperately reaching for a wing would be burned into Cap’s psyche forever. It would play in his head, over and over again, as a terrible reminder of the risks inherent to superheroics.
For many years, that was the extent of Bucky’s life and times.== TEASER ==
A few years back, Cap started fighting a villainous relic of the Cold War named the Winter Soldier. As it turned out, this nasty "wetworksman" was actually his long-lost sidekick. The kid’s barely-living body had been picked up by a Soviet submarine who subsequently revived him, brainwashed him and augmented him through the USSR’s own equivalent of the Super-Soldier Program. With a new cybernetic arm to get business done, Bucky was like a Manchurian Candidate who was “on” all the time. The Reds had him do their dirty work throughout the intervening decades, pulling him out of deep freeze whenever they needed a covert assassination.
Through the magic of the all-powerful, though-actualizing Cosmic Cube (an object of power which factors prominently into THE FIRST AVENGER,) Cap got Bucky to snap out of it. Wrought with guilt over all the crimes he’d committed as the Winter Soldier, Bucky nevertheless put aside his shame and reluctance and finally took on the mantle of Captain America while Rogers was believed dead. Even after Cap returned last year, he kept the identity while Steve operated as the Super Soldier.
Recent developments are starting to change who’s wielding the shield, though.
Bucky’s time as Soviet assassin wasn’t the only thing that was recently revealed about him. A “retcon” recast the once relatively-innocuous kid as an older (but still younger than Cap) junior special forces commando who’d get his hands dirty in battlefield business Rogers was too hesitant to take care of. He’s also had a long, on-and-off romance with the Black Widow.
Judging by the trailers, Bucky’s depiction in the forthcoming movie seems to be hewn a little closer to these later retcons as well as to some facets of his incarnation in THE ULTIMATES. He and Cap will be friends before the war - - with Bucky even protecting the frail Mr. Rogers from bullies - - and it’ll be his enlistment in the army that stokes the greater fire in Cap's belly to join the war effort. It also doesn’t look like Bucky will be wearing a domino mask or a bright blue outfit like he has traditionally. Instead, he’ll be a member of Dum Dum Dugan's elite combat unit, the Howling Commandos.
Though the Winter Soldier story might seem too involved for the Marvel Cinematic universe, Sebastian Stan (the actor playing Bucky) has reportedly signed on for six sequels. Unless those appearances are all going to be flashbacks, I imagine he’ll be showing up in some new guise after the conclusion of this first movie. Who knows? Maybe he and the movies' retooled Natasha Romanova will become an item eventually.