Steve Rogers’ best friend Bucky Barnes was thought to have died in World War II. However, he managed to survive the fall from Zola's train, though he lost his left arm. He was found by HYDRA soldiers and taken to a secret laboratory. The HYDRA scientists brainwashed him, replaced his lost arm with the robotic one, and gave him a serum similar to the Super Soldier Serum. Thanks to this serum, Bucky remained young until the 21st century, eventually becoming a brainwashed, mega-enhanced mercenary terrorist known as the Winter Soldier.
It is highly likely that while Bucky was captured, Arnim Zola was experimenting with an alternative to the Super Soldier Serum (which explains his delirious state in Zola's lab when Steve Rogers found him), thus granting Bucky the ability to survive his fall from the train in 1944. The hardened Winter Soldier has no memories of his former identity and that suits him just fine — he is simply a killing machine.
Powers and abilities
After Bucky fell into a trench in an ice mountain, he was saved and given a metal arm, along with some other enhanced abilities.
Abilities
Enhanced Strength: The Winter Soldier has strength that is even superior to Captain America. Much of his strength is due to his cybernetic left arm.
Enhanced Speed: The Winter Soldier's speed is also comparable to Captain America's, who had difficulty catching up to him.Winter Soldier's left arm is a cybernetic prosthetic with superhuman strength and enhanced reaction time.
Cybernetic arm: Winter Soldier's left arm is a cybernetic prosthetic with superhuman strength and enhanced reaction time. The arm can function when not in contact with Barnes and can discharge bolts of electrical energy from its palm. The arm can discharge an EMP causing electronics to either shutdown or become useless.
Skills
Master Combatant: The Winter Soldier is a highly trained martial artist and excels at improvisation and weapons handling. His time as the Winter Soldier helped to further hone his skills, making him the equal to his predecessor in combat skills and an expert assassin and spy.
Expert Marksman: Bucky was a remarkably precise shot, even prior to being turned into the Winter Soldier.
