Having a friend, even if it’s just a kitten, makes the narrator feel braver and refuse to go with Ursula. When the narrator thinks that Ursula is the adult world as a whole, it suggests that the narrator’s distrust of adults isn’t just about the people—it’s about the world they’ve built, which he inhabits, but where he has no power. Even though Lettie’s friendship helps the narrator feel braver, he still sees her as more or less a normal kid. This means that in the narrator’s mind, Lettie has no real standing when she argues with Ursula.