What I was thinking with him when the Earth kindom get liberated they find him in some of the cells beneth the city basically a physical and emotional wreck and Katara wants to bring him into take care of him, and Zuko feeling a little bad for the guy agree's, while Azula and Ursa in a rare moment of agreement decide that their are other uses they could have for Jet. Now it was just convincing Zuko to go for it.