Winter Soldier originates as James Buchanan 'Bucky' Barnes which has been both friend and side kick of Captain America . There is an accident that leads him that almost killed him and brought back from supposed death and then he got brainwashed as and assassin. From there he become the Winter Solder which has developed his combat and martial arts skills.
image sourceHe has also super human strength with cybernetic left arm. A Skilled assassin and a spy he can easily accomplished assigned missions.
He did join the Avenger after Captain American convinced him to join the Team.