Who is the Sleer?

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.

The Sleer is a creepy looking character, to say the least. A triple-headed snake-like creature, he's the oldest inhabit of the graveyard, where he's lived since before the ancient Romans arrived in Britain. Thousands of years ago, the Sleer used to be the servant of an old druid, or pagan priest, and his job was to guard the treasure—a brooch called the Snakestone, a knife, and a cup. But since his master left him, the Sleer has had no one to serve. That's what makes him especially vulnerable, despite his fearsome appearance, to being enslaved by the evil Jack Frost, who wishes to uses the Sleer for his own wicked ends. The Sleer was specifically created to protect his master's wealth from the grasping hands of the greedy and ruthless. Yet Jack Frost wants to use the Sleer to protect himself against those such as Bod who wish to rid the world of this malevolent creature. But the Sleer will never recognize Jack Frost as his master, as Jack discovers to his cost.