Who he is: The Winter Soldier, a.k.a. Bucky Barnes
His power: The Winter Soldier has enhanced speed, strength, stamina, and durability, though the extent of his chemically enhanced powers isn’t clear. His greatest asset is a robotic arm possessing force far greater than most any strength, even challenging Captain America’s.
His story: As seen in Captain America: The First Avenger, Bucky Barnes was Steve Rogers’ best friend, growing up with him in Brooklyn before going off to serve. Bucky got kidnapped by Hydra during World War II, prompting Rogers to grab his shield and stage a one-man rescue mission. Rogers saved Bucky and a bunch of other soldiers, including future Howling Commandos member Dum Dum Dugan. Unfortunately, during one of the Howling Commandos’ missions, Bucky fell from a moving Hydra chain and was presumed dead.
He survived, though, and while the next few decades are a blur to both the audience and Bucky, we know from The Winter Soldier that he was saved by the Soviets and/or Hydra. Having lost an arm, he was given a high-tech prosthetic by the Soviets/Hydra and then brainwashed into becoming an emotionless assassin who was kept cryogenically frozen when not in use. Under Hydra’s control, Bucky—now The Winter Soldier—killed a bunch of people, including Howard and Maria Stark. When Hydra made its big play and revealed that it had taken over S.H.I.E.L.D., Captain America realized his old friend was still alive and tried to force him to regain his memories.