In the story, the Sleer is a malevolent, serpent-like creature. It is also the oldest supernatural being in the graveyard. According to the text, the Sleer is a guardian. It watches over the treasures of its previous master while awaiting that master's return. This serpent-like creature also has three heads and three necks.
Its faces comprise an amalgamation of human and animal parts. Most disturbingly, the faces are covered in purple patterns, with swirling indigo tattoos.
Bod, of course, is wary of the Sleer. In the story, the Sleer asks Bod to be its master. The creature promises Bod everlasting protection from the dangers of the world if he acquiesces. However, Bod isn't interested in taking the place of the Sleer's previous master. He is more interested in finding out who killed his family.
Later in the story, Bod capitalizes on the Sleer's obsessive need for a master to dispatch Jack Frost, the evil man who killed Bod's family.