The red-haired nurse is super sketchy looking. And her last name doesn't help. She seems to be putting the moves on Sal's dad. But sometimes, things aren't always what they seem, so Mrs. Cadaver might not be as evil as Sal thinks.
For most of the story, we only know Mike as the crazy Lunatic who haunts the Winterbottom household and who might be responsible for kidnapping Mrs. Winterbottom. Just like we come to realize that Mrs. Cadaver isn't such a baddie, we also realize that the Lunatic's name is really Mike, and he's really just looking for a family. It's funny, then, how both our antagonists aren't really antagonists at all. So who is?
Death seems to be the most powerful antagonist in this story: it's what causes the most conflict for our protagonist. Death is responsible for taking Sal's mother and grandmother and temporarily robbing Sal of her happiness. However, in coming to terms with death, Sal is able to grow and move on. It's not as though she's conquered death – no one can – but Sal has learned to accept it as a part of life.