Here's what you need to know about him:
* Appearance: He has three heads, each with a different expression (usually snarling, drooling, and barking).
* Personality: He's known for being ferocious and terrifying, with a fierce bark and a love for chewing bones.
* Purpose: His job is to prevent anyone from leaving or entering the Underworld without permission from Hades, the god of the Underworld.
* Encounter: Percy, Annabeth, and Grover encounter Cerberus in the Underworld during their quest to clear Percy's name and prevent a war between the gods.
Cerberus plays a pivotal role in the story, forcing Percy to face his fears and demonstrate his bravery. He's a classic mythical creature that adds a layer of danger and excitement to Percy's adventure.