Morg is the main antagonist of Billy and Mandy vs. the Martians. He is a magical alien skeleton who works as the reaper of the planet Mars, a planet he also rules as the evil emperor over the zombies on the planet. His goal is to eliminate and collect the scythes of the other reapers in the solar system to dominate the universe.
To begin the first phase of his conquest, Morg appears disguised as a human lawyer to Grim, Billy and Mandy to inform them that Grim's friendship that he swore to Billy and Mandy would only be valid on Earth. He proposes that Grim leave Earth to be free. Grim accepts and heads towards the martian spacecraft where Morg, reveals his identity as the reaper and Emperor of Mars, beginning a friendship between the two deaths. Later, Morg convinces Grim (by reminding him of the torment he went through with Billy and Mandy) to join him in his plan to rule the Earth. Morg and Grim prepare to conquer Earth with his army.
Afterwards, Morg begins the second phase of his plan, find a human with the most stupid brain with which to control the zombies of planet Earth through stupid thoughts. They then try Morg's Brain-Caster with several residents of Endsville to find the perfect subject, but none are dumb enough. After Grim suggests that they use Billy, Morg tells him to stop thinking about them and proposes that they play a game for fun. Next to the game room, Grim discovers a room where the ashes and scythes of the other reapers of other planets lie. Morg tells Grim that they were all vaporized by accident, and that he was better than all of them. Grim pretends to trust Morg again.
Later, Morg discovers that Billy, Mandy and Irwin, seek to stop him, so Morg sends his zombies to capture them and bring Billy to put on the Brain-Caster while sending Mandy and Irwin into a pit with a dangerous creature. Later, after seeing Grim help Mandy, Morg decides he is no longer needed, and sends the inhabitants of zombified Endsville after them. However, Billy's stupidity makes the zombies dance. In an attempt to solve the problem, Morg accidentally breaks the Brain-Caster, returning Earth to normal and freeing Billy.
Finally, Mandy, Grim and Admiral Wolverine Lightningbolt corner Morg, who tells Grim that he'll always be at the service of Billy and Mandy, to which Grim states that he prefers to be with them. Mandy then proposes that Grim convert Morg, into his own servant, but Billy presses a button that launches Morg into space. Finally, Morg swears revenge before being sucked into a vortex.
- The name, 'Morg', is phonetically identical to "morgue", the area of a hospital where corpses are kept.
- Morg can be seen in Grim's memory in Wrath of the Spider Queen.