Cadogan

Sir Cadogan is the name of a knight in a portrait hanging on the seventh floor. Harry Potter, Ron Weasley, and Hermione Granger first encounter him on their way to their first Divination class; he guides them to North Tower. Sir Cadogan is widely considered to be insane by the students.

In Harry Potter and the Prisoner of Azkaban, after Sirius Black attacked the Fat Lady in an attempt to enter Gryffindor Tower, Sir Cadogan was appointed the guardian of the tower until the Fat Lady's portrait was repaired. Several students complained about his constant changing of the passwords &mdash; he was known for thinking up extremely complicated passwords and changing them several times a week &mdash; but Professor McGonagall, Head of Gryffindor House, ignored them. However, when Sir Cadogan allowed Sirius Black entry to Gryffindor Tower, he was promptly fired and the Fat Lady returned to her job.