He is the closest thing in the story to a totally good character. Hard working, honorable, loyal, and fair, he is equally comfortable showing both his raw, physical strength and his gentle, patient, emotional side. He is compassionate to the convict who stole his food, as well as to the memory of his alcoholic, abusive father. He recognizes Mrs. Joe's strengths, remembers her better times, and wants to protect her from the suffering that his mother endured with his father. In him, there is deep intuitive wisdom, inner peace and acceptance, dignity, and a basic sense of what is right and what will cause heartache. Even when treated poorly by Pip, he shows unconditional love and comes to Pip's aid when needed. His function in the story is to love Pip, be a father to him, and show him the path to dignified manhood.
Dickens keeps him from being a sickeningly sweet person by giving him the flaws of no education, no polish, and failing to better protect Pip from his sister when Pip was a child. Yet, to his credit, Joe himself expresses his realization of that and does what he exemplifies best. He takes responsibility for himself and consciously chooses his actions. He is not ruled by passion or illusion.