Grum was one of the soldiers tasked by King Gedol to keep the opossums in check after the lizard tyrant brainwashed them. Like most foot soldiers of Gedol, he is shown to be a dimwit, just following orders, though he's at least intelligent enough to pilot the tall E26 mecha.
When Sparkster returned to Zephyrus, Grum was ordered to attack the rocket knight. Despite Paerie's orders not to do anything to damage Sparkster's armor, Grum attempted to vaporize Sparkster with his E26 mecha's flamethrower. Sparkster managed to avoid Grum's attack and stabbed his sword into Grum's mecha, causing it to explode. Grum was likely killed in the explosion.