Argus Filch is the caretaker of Hogwarts. While he is not an evil character, he is ill-tempered, which makes him very unpopular with the student body, and occasionally with other staff. His knowledge of the secrets and short-cuts of the castle is almost unparalleled, except perhaps by the Weasley twins, and other users of the Marauder's Map. He tends to favour almost sadistically harsh punishments, leading to his alliance with Dolores Umbridge. He has an obsessive dislike of mud, animate toys, and all other things that might interfere with his creation of an immaculately clean and orderly Hogwarts.
Filch is revealed to be a Squib in Harry Potter and the Chamber of Secrets when Harry Potter accidentally discovers that he is trying to teach himself basic magic from a Kwikspell correspondence course. Nonetheless, Filch is at least able to use wizardry devices that have their own innate magic, such as the Secrecy Sensor used in Harry Potter and the Half-Blood Prince. In the Half Blood Prince, Harry Potter and Hermione Granger suspect that he is in a relationship with Irma Pince, the librarian at Hogwarts. In Harry Potter and the Deathly Hallows, the final book, when the school starts preparing itself for the Battle of Hogwarts, Filch is seen grumbling that students are out of bed. He is later seen overseeing the evacuation of younger students.
Filch has a cat named Mrs. Norris to whom he has a particular and possessive attachment. She acts as a hallway monitor or spy for Filch. If she observes students engaging in suspicious activity or out of bed after curfew, she finds Filch and he arrives in seconds. She has been known to follow Hagrid everywhere he goes in the school, apparently under Filch's orders. According to J.K. Rowling, there is nothing particularly magical about Mrs. Norris, other than her being "just an intelligent (and unpleasant) cat." It's the ambition of many Hogwarts students to "give her a good kick". In the Chamber of Secrets incident, Mrs. Norris is temporarily petrified by the Basilisk, which causes Filch extreme distress.