In the Marvel Cinematic Universe, Bucky Barnes was a close friend to Steve Rogers. He was recruited to become a member of the Howling Commandos before plummeting off a train and presumed dead. Though he survived and began a HYDRA soldier under the alias, Winter Soldier. After Steve Rogers confronts him, he becomes a close ally during the response to the Sokovia Accords.
In the Earth-616 universe, Bucky Barnes was the child sidekick to Steve Rogers. He joined Captain America in multiple missions, before being presumed dead after he tried to defuse a bomb. Later, it became apparent that Bucky was resurrected as a soviet assassin, under the code name the Winter Soldier. He later regains his memories before briefly taking the mantle of Captain America.