Billy is Jimmy's brother, who is taller and larger in size than Jimmy is.
In "The Reefinator", Lo misguidedly invites the hulking Billy to a pre-planned fight between Reef and Jimmy. During the fight, Billy and Jimmy gang up on Reef and intimidate him to the point where he attempts to no avail to beg his opponents for mercy and escape. The fight ends when Billy and Jimmy attempt to pounce on Reef, but collide into each other instead.