Sirius Black.
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What is the meaning of "Until fat lady sings"?
There is no person, ghost, or portrait that guards the Slytherin common room (actually called Slytherin Dungeon). It is hidden behind a wall, and a password must be spoken to the wall in order for it to move aside.
The password for the fat lady. However, it was changed often, especially after the fat lady got attacked by Sirius Black.
Harry Potter did not get out of Gryffindor, he was sorted into this house and was part of it until he left school. To enter and exit the Gryffindor common room, he went through the portrait of the Fat Lady. To enter he had to give her the correct password.
represents the sitters profession