Harry Potter and the Chamber of Secrets
by J.K. Rowling
Mrs. Norris is a cat belonging to Hogwarts caretaker Argus Filch. She seems to act as a kind of spy for Filch, somehow informing him when students are breaking the rules or dirtying the castle. Mrs. Norris is the first victim of the Slytherin monster; she is found on Halloween night hanging by her tail from a torch bracket, stiff as a board. Filch thinks that Harry is the one who Petrified her because he knows that Filch is a Squib (a non-magical person with magic parents). Professor Dumbledore reassures him that this is not the case and Filch seems to feel better, but he still skulks "red-eyed through the corridors, lunging out at unsuspecting students and trying to put them in detention for things like 'breathing loudly' and 'looking happy'" (9.62).