Lucky is the elderly slave of Pozzo, an elderly man himself. He is physically whipped and prodded onto the stage and forced to dance and sing like a trained monkey. If Vladimir and Estragon are waiting for hope in the form of a person named Godot, Lucky is the arrival of bitterness and sadness.
Oddly, Lucky does not lash out at Pozzo, but is eager to do what is asked of him despite the pain and sadness it causes him. In this, Lucky, as a character is a paradox. Why does he act kindly toward a man who grossly mistreats him?
Interpreting Lucky's character depends on which approach you take. For example, a religious intepretation could have Lucky represent faithful because he follows Pozzo around when Pozzo mistreats him and when Pozzo is nicer to him.
Another interpreation is that Lucky is actually lucky. He doesn't have to make any decisions because he is ordered to do everything he does.