William "Billy" Idiot is a main character in the cartoon series, The Grim Adventures of Billy and Mandy. He is Mandy's best friend; He is a slow-witted, happy-go-lucky child. He has an extremely low IQ of -5, having been outperformed by a shovel and two candy bracelets on an IQ test. He has an extremely large, pink nose inherited from his father, Harold, wears a blue and white-striped shirt and covers his ginger orange hair (which is inherited from his mother, Gladys) with a red cap. His main affiliations are Grim and Mandy. Mandy is Billy's best friend, though she treats him more like a servant than a friend. It has been hinted several times that Billy may have underlying feelings for Mandy that even he (in his stupidity) may not fully realize. Billy is far kinder to Grim than Mandy is, and while he almost always goes against the Reaper's advice and uses him as a plaything, he appears to genuinely like him and often tells Grim that he's his "best friend". He appears in the first chapter, as Grim saved his life (amongst many others), during the Omega Event. The rest of the chapter, he was said to play D&D with Irwin.
Boogeyman is a recurring villain in the cartoon series, The Grim Adventures of Billy and Mandy.An old arch-enemy of Grim's since junior high school, and a former school bully, Boogey played pranks on Grim throughout his school career (one such prank involved publicly humiliating Grim in front of the whole class by giving him a nuclear wedgie from off-stage whilst Grim was reciting a scene from Hamlet; Grim never lived that incident down since everyone called him "Wedgie Shakespeare" afterwards). Boogey is, in fact, the Boogeyman himself, and can shapeshift in order to scare people. Boogey is Grim's rival in scaring people, but usually fails in doing so. Being constantly frustrated by his inability to scare modern-day children (he blames cartoons, video games and for some reason, the tuba), Boogey visits Grim in the episode Bully Boogie. Mandy suggests a challenge of a scaring contest, which Boogey promptly loses, and is banished to the Pit of Terror/Nightmare Realm. Ironically, it is the same dimension to which he sent Billy, one filled with spiny plants and a creature similar to the plants. Grim's past with Boogey was brought up several times in the first chapter.
Chaos, voiced by Matt Frewer, is a villain from Aladdin (TV Series). He is a winged blue cat who desires that life be unpredictable, full of surprise and change. He was mentioned by Grim and was the one who orchestrated the events that led to Grim becoming friends with Billy and Mandy.
Eris is a recurring character and sometimes villain, in the cartoon series, The Grim Adventures of Billy and Mandy. Eris, based on the Greek deity of the same name, here is portrayed as a curvaceous and beautiful, but vindictive, blonde woman with a gap in her teeth. She delights in causing all kinds of chaos, and does a multitude of destructive things. She possesses the powerful Apple of Discord, a golden apple which can transform into any shape to perpetuate chaos (though Billy calls it a "magic banana"). Her accent and behavior are prone to sudden shifts, from a stereotypical valley girl to a refined British woman, parodying Madonna's change in image over the years. It is revealed in "Wrath of the Spider Queen" that Eris attended the same school as Grim, Velma, Nergal, Lord Pain, and Boogey. Grim used to have a crush on her during earlier episodes, but later changes his mind, claiming her to be a "psychopath." She is also in love with Hoss Delgado. Eris was briefly mentioned by Grim as he described Mandy's taste for chaos to be so great, it even outdoes Eris'.
The Firstborn is the main antagonist of Clive Barker's Jericho. It was created by god before Adam and Eve, but He realized that it was too powerful, but couldn't destroy it, only banish it to The Pyxis. However, It has tried 6 times to break into our universe, and each time the Department of Occult Warfare sends a group of mystics to sacrifice themselves and seal the breach. It is Prophecised that the seventh attempt will be it's last. However, the final Jericho squad managed to Highly damage it before it's former apprentice, Arnold Leach, carries it into a tunnel of pure/light energy where a flash of burning light occurs.It is unclear if they were both destroyed or if they have teleported somewhere. The Firstborn's creation was mentioned as the cause of the Great Divide. He was created by God, in fear that the forces of Darkness would create a being that would dominate the Earth. This failed however, leading to a creation that was eventually imprisoned into Pyxis Prima.
General Immortus is a villain from the cartoon series, Teen Titans. As implied by his name, General Immortus is an ancient military commander who is knowledgeable in every combat strategy ever conceived, mainly by being involved in every major battle in history personally. General Immortus was mentioned as being an Unreapable, and therefore immortal.
Vandal Savage is an immortal warrior who intends to conquer the planet. Thousands of years ago, as a caveman during the Prehistoric Era, he was exposed to a meteorite that enhanced his physiology. He was the inspiration for the biblical figure Cain, having committed the first recorded murder, making him an important figure in the Religion of Crime. Throughout the ages he has been an influential figure in history, manipulating many of its greatest conflicts. In the modern era he regularly fights against the Justice Society of America, his greatest enemies. Master of planning and many forms of combat, having survived and adapted through countless centuries, Savage is an extremely dangerous and formidable opponent. Although his campaigns frequently meet with heavy resistance, he is resilient in the knowledge that he will outlive all those who oppose him. Vandal has fathered thousands of children, but the only one he truly considers his heir is his daughter Scandal Savage. Vandal Savage was created by Alfred Bester and Martin Nodell. He first appeared in Green Lantern #10. (1943) Vandal Savage was mentioned to be an Unreapable, meaning the Grim Reaper can't claim his soul, rendering the man immortal.
Velma Green the Spider Queen is the false main antagonist in Billy & Mandy: Wrath of the Spider Queen, having moved from The Spider Planet in Galaxy Omega 9 during her and Grim's childhood. She is voiced by Kari Wahlgren. Velma was once Grim's best friend during their days at Doomsday Junior High together, but their strong friendship later dimmed when Velma thought that Grim had cheated to be the Grim Reaper (in reality, Grim had tried to help Velma win.) She was Jeff's fiancee and bride-to-be, but Billy told Jeff she wasn't good enough for him and Jeff canceled the wedding. The history between Grim and Velma is briefly mentioned in the first chapter.