Who is the sleer?

The Sleer is a fictional creature from Neil Gaiman's "The Graveyard Book." It is a supernatural being that guards a graveyard's secrets and treasures, manifesting as a fog-like presence. The Sleer is ancient and powerful, drawing strength from the fear it instills in others. It plays a significant role in the protagonist Bod's journey, symbolizing the darker aspects of death and the unknown.