Who is the protagonist and antagonist of the book Staying Fat for Sarah Byrnes?
Eric "Moby" Calhoune and Sarah Byrnes are the protagonists in the story, with the emphasis on Eric, as Sarah is in a mental hospital and catatonic for a significant part of the story.
As Sarah has been hospitalized, her best friend Eric has to try and find out what has caused her mental illness and possibly find a way to help. They are aided along the way by friends and some family and teachers at Eric and Sarah's school.
Virgil Byrnes is Sarah's father, and he is the clear antagonist. Later in the story, it comes out that Virgil had abused Sarah when she was younger, and that her mental illness is actually a cover because Sarah has become increasingly afraid that her mentally unstable father might try to kill her.