BJ's reaction to Lydia vanishing right in front of him. Yeah, he definitely terrified Hugo. But it is nice to see how much he cares about Lydia and it is pretty clear what he'll do to get her back. Specifically, BJ promises not to kill Hugo after his magic ritual goes horrifically wrong. He says that instead, he will wait "ever so patiently" while Hugo finds a way to fix the problem.
BJ: Because if you don't, I will personally see to it that every moment of the rest of your life is a completely hellish nightmare until you give up and die in self-defense!
Lydia taking out Erebos in the most logical method to defeat a Living Shadow: with lots of really bright light.
Beetlejuice's absolute fury when he figures out what Vasile's plan for Lydia is.
A meta example for the author herself - the Fairy Godfather has been so embraced by the readers, a few of them actually mistook him for a canon character, and were asking which episodes he was in so they could watch them.