Cerberus

Cerberus is Paul Gekko's Pet Puppy.

Background
When Paul Gekko is Eighteen, he received a three headed puppy. Paul Gekko fed the beast with massive Pizza.

Trivia

 * The Cerberus is mostly based on the Greek mythololgical monster of the same name, they are both monstrous three-headed dogs and their job is guarding the entrance of the Underworld.