Jack is the fun uncle. You know, the one who brings you firecrackers and teaches you how to shoot a gun. (No? Just us?)
He may be a joker, but, unlike Atticus, he's not always fair. Scout points this out when he punishes her for beating up Francis without letting her tell her side of the story; when he hears what she has to say, he's ready to go beat up Francis himself, and possibly Aunt Alexandra to boot. Uncle Jack's willingness to admit he was wrong, and his loyalty in keeping his word to Scout (he promises not to tell Atticus why she was fighting Francis), put him in the category of adults who treat children with respect (along with Atticus and Miss Maudie).