Bucky was Captain America's partner during World War II. After being brainwashed by the Red Skull, he became an agent known as the Winter Soldier but later broke free and became a freelance mercenary.
In the episode "Ghosts of the Past", Bucky infiltrates Avengers Tower and kidnaps the Red Skull. His origin is briefly recounted, with it stated that he was thought dead in World War 2, only for the Red Skull to transform him into a brainwashed assassin. He now seeks to have his revenge by killing the Red Skull, which brings him into conflict with the Avengers, who need the information the Skull has on Thanos and the Infinity Stones. He is forced to flee after his plan is thwarted.
He is a true believer in Captain America and is quite a patriot. He usually makes jokes on opponents like the time he made a joke about Baron Strucker about his "hair issue" being bald and all, which angers Strucker. However, Bucky is also ashamed of what he has done as Winter Soldier. Bucky has shown signs of aggression whilst depressed and has also redeveloped a relationship with Black Widow. But he is also sarcastic and makes jokes like his light-hearted self.
As the Winter Soldier he has super strength and quick reaction time and the ability to produce EMPs, as well as jammers for metal detectors and x-ray machines, due to his cyborg left arm. Even when this arm is removed, Winter Soldier can mentally control it from a remote location. His time as a covert Soviet agent has further honed his skills giving him a combination of mix fighting styles for use in close combat. As the Winter Soldier, Barnes became an expert assassin and spy and performed many behind the scenes assassinations without getting caught making some of his kills look like accidents. Thanks to Nick Fury, Bucky's bionic arm is now covered in a flesh like material making it both look and feel like a real arm.
As the Winter Soldier, James's missing arm was replaced by a cybernetic enhancement. As his career progressed as Captain America, Nick Fury found it necessary to upgrade his arm to be fully synthetic in order to look and feel real.