Fat Lady

From Hpwiki

The Fat Lady is the portrait that guards the Gryffindor tower in Hogwarts. To enter, you must say the proper password. The first ever password mentioned for the Griffindor Common Room was 'Caput Draconis'. Like all figures in all magical pictures, she wanders off every now and then. During these periods when the Fat Lady has taken temporary leave of her portrait, students and teachers alike are unable to gain entry to the Common Room.

In Harry's third year, Sirius Black had escaped from Azkaban and attacked the Fat Lady, viciously slashing her portrait in an attempt to gain entry to Gryffindor Tower to find Peter Pettigrew, forcing her to run out of the portrait and hide in another. She refused to return until she was given heightened security in the form of armed security trolls, and in the interim until her return, was replaced by Sir Cadogan, much to the annoyance of the students.