Argus Filch, more commonly known as Mr. Filch, is a minor antagonist in the world of Harry Potter. He is the Caretaker of Hogwarts School of Witchcraft and Wizardry, a job which he has maintained for several decades. While his duty is primarily the care of the castle, duties that include custodian work as well as security, Filch is greatly interested in, and enjoys, dishing out punishments to the students. While the old ways of severe, and violent punishment were eliminated by Headmaster Albus Dumbledore, Filch lamented this, and was all too eager to continue them again during the cruel reign of Dolores Umbridge over the school. He is the archenemy of the poltergiest Peeves.
During the Chambers of Secret incident which some students including Hermione Granger and his cat been petrified by the Basilisk, he blames Harry Potter for the attack instead. His actions against Harry also made Rubeus Hagrid to become worried and hostile to Filch until he finally realized that it was the Basilisk who did really petrified his cat and the other students.
It should be noted however, while Filch was generally a very unpleasant person and quite the sadist, even more disturbing considering his cruelty was aimed towards those underage, he was still ultimately loyal to the school. He assisted in the evacuation of the students during the Battle of Hogwarts, and even took part in the Battle himself. When the Battle was over, he immediately returned to his caretaker duties, and was seen cleaning up the massive mess left behind in the Battle.