Adventure Time attracted a lot of viewers due to its unique design, quirky characters, and diverse storylines. However, as cute and colorful as its world may look on the surface, any Adventure Time fan can attest to how that only hides a deeper, more complex history, and the same thing can be said for its characters.
As adorable as the cutest character can seem, there's a good chance that they're hiding some supernatural power or demonic entity within them. Deception is the name of the game, as Finn and Jake have encountered all sorts of enemies and bystanders whose looks completely undersell their power.
King Worm
Adventure Time is host to a variety of cute talking animals, magical creatures, and random, regular critters that are still pretty cute. However, as one episode proved, Adventure Time's animals are heavily changed and can hide sinister things under their beady eyes.
One of the best examples of this is the King Worm. While worms are typically at the bottom of the food chain—as is made evident in the episode "Food Chain"—King Worm showed his prowess by using his hypnotism powers to nearly trap Finn and Jake within a seemingly endless dream state.
Fire Wolf Pup
The episode "Thank You" set out to show how, no matter how divisive people's differences are, they can still care and connect for one another. This is illustrated through the growing friendship between this episode's Snow Golem and one fortunate fire wolf pup.
While Snow Golems are plenty strong and frightening within their own right—when they're not shopping for pears, that is—they don't really stand a chance against their natural predator, the fire wolves. These barking pieces of charcoal come from Adventure Time's volcanoes and can be vicious when aggravated. However, one pup took a liking to a snow golem and depended on it to find its family again.
Dirt Beer Guy/Root Beer Guy
Root Beer Guy may look like another delicious citizen of the Candy Kingdom, but he has incredible durability and strength brought about by events straight out of Frankenstein. Root Beer Guy became prominent after Princess Bubblegum promoted him to captain of the Banana Guards in "Root Beer Guy."
Said position would sadly cost him his life after Maja supposedly killed him, but he'd return as Dirt Beer Guy after a stray bolt of lightning resurrected him. He now lives as a plant-like zombie with monstrous strength who accidentally destroyed his own home after trying to reconnect with his wife.
Ghost Fly
It's common for a lot of people to swat at flies, especially around dinner time. While they have to eat, they can become a nuisance when trying to keep food nice and clean. However, killing flies doesn't always fix the problem. As the episode "Ghost Fly" showed, flies can be an even bigger problem when they come back from the dead.
After Jake kills a fly trying to get his soup, it becomes a ghost that threatens to kill Finn and Jake with a ghost scythe. Peppermint Butler tries to help but has to run away in fear when the Ghost Fly steals his tools. Finn even gets possessed after dropping his guard, and, in contrast to the fly's typically cute and cartoonish demeanor, becomes a horrible monster similar to what's seen in the movie The Fly.
Jake
Next to Finn, Jake is one of Adventure Time's best characters. However, just beyond being a pretty face or a good friend, Jake is also one of the most powerful characters in Adventure Time. As the son of two paranormal investigators, Jake was already pretty strong when facing supernatural threats.
However, he's much more than that thanks to his incredible abilities. Initially billed as a "Magic Dog," Jake the dog is actually a talking dog/alien hybrid. It's from his alien side that he gains his miraculous ability to grow and shapeshift into incredible forms and proportions and, after learning about his origins, change color.
Me-Mow
Me-Mow is a villain who's managed to be a considerable threat to Finn and Jake despite her small size and darling design. As part of the Guild of Assassins, Me-Mow's first run-in with Finn and Jake happens when they get in the way of her assassination of Wildberry Princess. She starts targeting Jake, using her small size to get away with various attempts at sabotage.
Jake, however, is able to eventually defeat Me-Mow and send her away with Wildberry Princess' help. Me-Mow made a surprising return in "Angel Face," as an adult-sized bounty hunter who, after misinterpreting BMO's cowboy game with Finn and Jake, manages to kidnap Finn. Me-Mow's most recent appearance has her fighting in the Gumbaldia army, presumably as another act of vengeance.
Peppermint Butler
Peppermint Butler is Princess Bubblegum's most trusted confidant whose dark side and magical abilities have likely kept the Candy Kingdom safe in secret. Due to unknown origins, Peppermint Butler has and is always destined to be a master of the dark arts, specializing in capturing, manipulating, and communicating with demonic entities. His actions have resulted in some of the scariest moments in the Adventure Time series.
This isn't always for good, as he's made enemies across demonkind. However, his supernatural knowledge has helped the main characters multiple times, though they do try not to ask too many questions. In the event they do, they might learn a little too much about a guy who's better off just being a nice, piece of peppermint.
Gunter/Orgalorg
According to Adventure Time's fill-in character for the devil/Marceline's dad, Hunson Abadeer, Gunter is by far the most evil thing he's encountered. While this seemed like a joke at first meant to contrast Gunter's otherwise loveable demeanor, Hunson actually saw something that no one else did.
As it turns out, Gunter was actually an evil ruler of the cosmos who used his incredible size and strength to toss entire celestial bodies. When he tried to harness the power of the Catalyst Comet, Abraham Lincoln and Grob Gob Glob Grod thwart him and send him to Earth where his body became compressed by Earth's atmosphere, leaving the tiny penguin that Ice King has treated like a chihuahua. As cute as his Gunter form has been, it's disappointing that alien Orgalorg didn't get used enough throughout Adventure Time.
Kitten
While Orgalorg himself is powerless within Earth's atmosphere, his offspring are far from futile beings. Kitten was first introduced in "The Chamber of Frozen Eggs" where Gunter's sudden ailment just turns out to be a pregnancy. When his egg eventually hatched, it gave way to the "horrifyingly beautiful" birth of Kitten.
While this could've just been another one of Adventure Time's surreal gags, Kitten would return in "Reign of Gunters" where it flew and used magical laser beams to help take down the Candy Kingdom.
Sweet P
Sweet P is the sweet and friendly adopted child of Tree Trunks and Mr. Pig. He's been used to be a caring and upstanding citizen who seemingly grew up to be another adventurer within The Land of Ooo. However, as his horns communicate to anyone unfamiliar with Adventure Time, Sweet P wasn't always a tender child.
He is, in fact, The Lich, the personification of nuclear death who was given life when Finn coated him in the life-giving blood of The Citadel's guardians. Though he may appear innocent as a fresh child, Sweet P still holds The Lich's same powers over destruction and consciousness, and has used that strength to combat thieves and even The Lich himself. As Sweet P has said, "I, too, am doom." Here's hoping that Sweet P is one of the characters who shows up in the Adventure Time Distant Lands special.