The Winter Soldier and Captain America have one of the closest friendships in Marvel Comics. Back in World War II, Bucky Barnes was the Robin to Steve Rogers' Batman. This relationship made it that much more intimidating when Bucky had to become the next Captain America.
In Captain America #25 by Ed Brubaker and Steve Epting, Steve was assassinated as part of a plot by the Red Skull. Despite Bucky and the Falcon's attempt to intervene, they were too late. Captain America was dead, leaving a gaping hole in the world. Filled with rage, Bucky decided to go after those he deemed responsible: Red Skull and Tony Stark. The Winter Soldier blamed Iron Man for his part in the recent "Civil War" storyline.
After a heated physical confrontation, Tony finally got Bucky to listen to reason. Evidently, Steve had asked Tony to honor two of his wishes after he died: first, to rescue Bucky from his violent life as the Winter Soldier and second, Steve wanted Tony to pass the identity of Captain America onto someone new. Killing two birds with one stone, Tony proposed that Bucky become the next Captain America. The Winter Soldier was hesitant, but, refusing to accept anyone else in the role, Bucky decided to honor Steve's dying wish. Of course, Bucky accepted this offer on the condition that he didn't answer to S.H.I.E.L.D. or the Superhuman Registration Act. As Captain America, Bucky received a brand new costume designed by Stark, along with the classic shield. Bucky also carried around a pistol, harkening back to his days as both Captain America's sidekick and the Winter Soldier. From there, the new Sentinel of Liberty made it his mission to take down the Red Skull. Along the way, Bucky fought the Skull's allies, such as Crossbones, Sin, Arnim Zola, A.I.M., and Doctor Faustus. At one point, Bucky even faced the Captain America from the '50s. This imposter was used by the Skull to manipulate public opinion, as the American people would faithfully listen to Steve Rogers. Of course, Bucky ultimately proved himself to the public by stopping a political assassination and exposing the other Captain America as a fraud.
During his time as Captain America, Bucky gradually developed several new partnerships. Bucky and the Falcon worked together just as Sam Wilson worked with the original Captain America. In Secret Invasion #6 by Brian Michael Bendis and Leinil Francis Yu, Bucky fought alongside heroes like Iron Man and Thor against a horde of Skrulls. Afterward, in New Avengers #48 by Bendis and Billy Tan, the new Captain America joined the New Avengers. Bucky even let the team use Steve's apartment as their headquarters. Together, the New Avengers fought against Norman Osborn and his "Dark Avengers." Most importantly, Bucky developed a relationship with the Black Widow. As the Winter Soldier, Bucky shared a history with Natalia Romanova, forming a deep connection between the two. When Bucky became Captain America, he and the Black Widow worked alongside each other as heroes and lovers.
In the pages of Captain America: Reborn by Brubaker and Bryan Hitch, Steve Rogers was revealed to be lost in time. Bucky worked with Steve's friends to bring him back to the present, and the two Captains defeated the Red Skull once and for all. Although he had returned, Steve let Bucky keep the mantle of Captain America, while he became the new Nick Fury. During this period, Bucky joined the official Avengers, giving Steve a break from his old job.
Yet Bucky couldn't escape his past as the Winter Soldier. Baron Zemo revealed Bucky's previous crimes to the public, sending the former Winter Soldier to trial. After his trial, Bucky was extradited to Russia and sent to the Gulag, as the Russian Ambassador held Bucky responsible for his crimes. Black Widow managed to rescue him, but by this point, Bucky's time as Captain America was drawing to a close.
In Fear Itself #3 by Matt Fraction and Stuart Immonen, Bucky was seemingly killed by Sin during an attack on Washington D.C. In truth, Nick Fury gave Bucky the Infinity Formula, faking his death. People like Fury were clamoring for Steve's return as Captain America, and Bucky's record was tarnished by his past crimes. With this in mind, Steve reclaimed his title and Bucky went underground on new missions as the Winter Soldier. In the end, Bucky's past was far too dark to carry the shining mantle of Captain America.