I like to think that Mohatu was really evil before he turned out good. After realizing what he had done to everyone and after he got a mate, he tried to be a better person, and managed to become the brightest star. ^.^
I have no backstory about the selfish lion, but I have a similiar OC that helps Mohatu come back to his senses. That OC isn't selfish though. Maybe he [the selfish lion] could be Ahadi's father, if Uru was really Mohatu's daughter? [or vise versa]