I don’t think the characters in this short story have too many redeeming qualities. In fact, that is the point. Joy/Hulga is spiritually as well as physically crippled. Her soul is as wooden as her leg, as O’Connor herself described it in her letters. Joy/Hulga is therefore a sinner in need of redemption, so perhaps the only thing you can say that is redeeming about her is that she is human. Her mother, Mrs. Hopewell, has “hope” and love for her daughter. She named her “Joy” after all, even though the surly girl changed her name to the ugliest one she could think of – “Hulga”. Nevertheless, Mrs. Hopewell, although an annoying woman, does love her daughter, but she is intimidated by her. There is not much redeeming about the Freeman family – the girls are vacuous and the mother is a monologuer and braggart. Finally, the Bible salesman is a con man and a liar. When he steals Joy/Hulga’s wooden leg, he has stolen part of her personality and her soul, but this reveals her deeper affliction for the first time. At this point in the story, the Bible salesman, in an ironic twist, reveals himself to be a representation of the devil. When he steals Joy/Hulga’s leg, it is at this point that everything is stripped away from her and she is humbled for the first time in her life. In O’Connor’s fiction, it is often when characters are brought so low that they are ready to be redeemed. In this story, however, we do not see it – we just see that Joy/Hulga is ready. All of her pride has been stripped away, so she is ready to receive grace and mercy, but then the story ends.
We also see the Bible salesman moving on, to do his evil to some other unsuspecting soul, so he deserves damnation, but we don’t see it. Mrs. Freeman and Mrs. Hopewell see the Bible salesman walking off, assuming he has tricked some of the black people into buying Bibles, while peeling "evil-smelling" onions. It's as if he has left his evil smell behind.
You might be interested to know that in her letters, Flannery O’Connor often said that Joy/Hulga was one of the easiest characters to create and that she related to Joy/Hulga more than any other character.