[FairyKing]

Now for the villains, a sinister nameless faction who works in the shadows to bring a return to the Dark Age when humans and monsters fought for survival.

- Doctor Grim, a lich and brilliant occult scientist who's the mastermind behind the monsters going berserk. He created a power-up potion that makes monsters stronger, but taking too much will fill them with uncontrollable rage. He presents himself as polite and reasonable, and seeks out other monsters who also want to hunt humans to try and convince them to join his cause willingly, but if someone refuses him he won't hesitate to trick, bribe or threaten them into doing what he wants, or just drug them with his potions. He was Sparky's mentor in the past, and used to work for the good of all monsters, but now his goal is to bring a second Dark Age which will wipe out humans and weak monsters alike, so that only the strongest monsters remain.

- Yuriko Miyagi, the ghost of Hikaru's younger sister, she was killed by a vampire and then resurrected by Doctor Grim and forced to work for him as part of a scheme to manipulate Hikaru. She's afraid of monsters despite now being one herself, and only fights when she has no other choice. She appears before defeated monsters and injects them with a regrowth potion to revive them as giants, and before discovering Grim's involvement the Monstrangers thought she was the mastermind. After being rescued by her brother and Drack, she technically becomes the sixth Monstranger, Monster White, but because of her fear of fighting she only joins in battle as a last resort.

- Zero, a zombie who prides himself on being Doctor Grim's "better than number one assistant." He's extremely loyal to Grim, but will bully or betray other members of their faction if he feels they're a threat to his position. He was originally looked down on by other monsters for being weak, but Grim experimented on him to give him more power, including the ability to rapidly clone himself, and he serves as footsoldiers to support Grim's schemes. His mind is shared between all the clones, so he can survive even if his original body is killed, but as a downside the more bodies his brainpower is divided between, the less intelligent each one gets.