James Buchannan "Bucky" Barnes was an ally to Steve Rogers and assisted him in various missions as a member of the Howling Commandos. During one mission he was seemingly killed in action by falling off a train on the side of a mountain.
However, he survived and was frozen. He was revived and received a metal arm, but lost his memory. He was trained to be an assassin, given the code name Winter Soldier, and sent on many missions.
He fought with Captain America who managed to restore Bucky's memories.