Bucky Barnes (Marvel Cinematic Universe)

James Buchanan Barnes is the tritagonist from Captain America: "The First Avenger" turned secondary antagonist from "Captain America: The Winter Soldier", but at the end, he redeems himself by

saving his friend Steve Rogers. Bucky will returns in "Captain America: Civil War".

Bucky was Captain America's best friend and helper during the Second World War, during a fight on board a train, Bucky will come presumed dead, but will be saved dall'HYDRA that will transform him into a super soldier.

in this, Bucky become enemy of Captain America, but when Steve will be life threatening, Bucky will save him.