Billy is the deuteragonist in the dark comedy series, The Grim Adventures of Billy & Mandy.
While the most innocent character of the three, he has been known to have moments of dangerously deranged, contemptuous, and irreverent behavior. Always optimistic due to being thoughtless to the point of extreme retardation (his I.Q. is -5) and schizophrenia, he is often a person to be taken advantage of. Despite having many moments of kindness he has also committed some rather atrocious acts.
In "Attack of the Clowns", he displayed insanity and an illogical fear of clowns which he confronted by developing a psychotic hatred for anything different albeit harmless. This ended with him butchering his best friend Irwin who accidentally ended up dressed as a clown. Another example is the violent behavior that he showed towards giant spider Jeff because of his arachnophobia. Foolishly and quite amusingly Jeff seemed to consider him his father and saved his life numerous times. This didn't stop the unstable kid from trying to murder him anyway.
One must not forget that Billy always had a tendency of becoming a pet to darker individuals than him. Apart from Mandy, he also became a henchman of the Brain-Eating Meteor and helped him consume the brains of the entire city, including his own parents. He seemed to show no remorse for helping Jack 'O Lantern to wreak havoc and almost helped Eris in the same way. He also tried to corrupt Skarr to go back to a life of crime on the grounds that its "cool". In the first season he wanted to have local bully Sperg killed in response to continuous abuse . In the episode "Whatever Happened to Billy Whatshisname?", he even threw another dumb kid through a portal to a horrible fate just because he took his friends, though in fairness the boy in question wasn't really dumb and was actively trying to steal Billy's life (even threatening to kill him if Billy didn't leave).