Protagonists

• Russell Wright (voiced by Mark Rendall)^[4][2] moves into a new house with his family, comprising his older sister Lou, and his parents (the father was only seen briefly in short scenes throughout the series). Thanks to Gus (and Cliff, technically), Russell discovers the passageway beneath his bed in the turret room (which he chose after unknowingly seeing Vernon's eye-looker probe in the window) and uses it to descend to Under. There he befriends Loopy and Vernon, and challenges Bob Wire for the crown, subsequently becoming Under's new king. Even though he's only 12 at the onset of season 1, he is a smart and righteous ruler of Under in contrast to other former rulers. The crown confers upon the wearer certain abilities, at least one of which is the ability to breathe underwater, as long as the crown is being worn. It is assumed these abilities work only in Under.
• Gus (voiced by James Kee)^[4] is a West Highland White Terrier, who can talk with a Scottish accent in Under (where dogs, cats, and possibly other Up animals are sapient). He is "doggedly" loyal to Russell and his friends, and is very brave, sarcastic, blunt, and gutsy, often taking on (and usually scaring away) much larger foes.
• Loopy (Adam Greydon Reid)^[4][2] is one of Russell's subjects, who displays effeminate and cowardly tendencies, such as being obsessed with butterflies, Shoosh-Bangs (fireworks), and the name "Gack" (which he wishes to name everything from pet animals to Up). He is part of the royal court as Under's First Buffoonist. He is also shown to be too stupid for a Brain Sucker to affect him. Loopy does come through for Under in his own special way: he has the inherent ability to tame almost any animal, from squumps to sky cows to various undersea creatures. Loopy and Vernon are best friends, despite their slapping and bickering now and again. In the series finale, he is revealed to be a member of The Secret Society of Nose Bellers.
• Vernon (voiced by Robert Tinkler)^[4][2] is one of Russell's subjects, who is an android built by the Clockmaker and is also part of the royal court as Under's figurer-outer and gizmologist. As he's made of metal, he is immune to most things that happen in the realm. Unlike Loopy, Vernon is a genius inventor, and their differences often lead to sibling-esque slap-fights.
• Ex-Princess Populah (Julie Lemieux)^[4] is a former princess, who used to be the ruler of Near Under but "gave it up" because it was "too much trouble" (as revealed in the episode "Ex-Princess Up a Tree"). This is reasonable as Near Underites are quite demanding and fickle, difficult for their leaders to rule. She is related to Auntie First. (Although everyone calls her "Auntie", First has confirmed her relation to Populah by referring to her as her "favorite niece"). She is also a blood relative of Cousin Tess (although everyone calls her "Cousin", in the same way as everyone calls Auntie First "Auntie"). She speaks with a Southern accent and is very athletic. She can breathe underwater naturally, without the need of the crown, as can Cousin Tess.

Antagonists

• Cliff (voiced by Robert Tinkler)^[4] is the former King of Under. Cliff is a bully who constantly picks on Russell because he wants to be king again. Sometimes, he attempts to sneak into Russell's room (and down to Under) while Russell away, usually being foiled by Russell's teenage sister Lou (Voiced by Stacey DePass) or his own stupidity. In the series finale, he temporarily succeeds in regaining the crown via a deal with Russell to defeat Gag Pune, but he abdicates it when he is unable to defeat the creature. He has a sister named Sissy who is affectionate towards Russell, which only makes Cliff angrier.
• Bob Wire (voiced by Cathal J. Dodd)^[4] is Cliff's former assistant, who has taken to trying to make himself king of Under by stealing the Crown from Russell. He is made out of coils of metal barbed wire (hence the pun on his name), and like Vernon, he is immune to most things that happen to him in Under. His plans generally tend to be rather badly thought out and self-defeating, but he usually bounces back when he is defeated. The few things he is good at are singing, high finance, and show business. He appears to be the main antagonist of the series, appearing in all episodes except for "Random Opportunity Knocks" and "The Museum of Tomorrow".
• Frags (Robert Tinkler and James Kee)^[4] are two small green creatures (there were several in the original few episodes) who are henchmen of Bob Wire. They tend to be extremely lazy and are generally somewhat more intelligent than their boss. They sometimes take the side of Russell or one of his companions. Their voices are high-pitched but randomly switch to low-pitched in a manner akin to croaking. Like their boss, they appear in all episodes except for "Random Opportunity Knocks" and "The Museum of Tomorrow".
• Auntie First (voiced by Marnie McPhail)^[4] is the narcissistic, self-absorbed, oppressive despot of Near Under. She is not very good at her job but maintains it nevertheless, generally through a combination of threatening bodily harm, bribery, and blackmailing. She is referred to as "Auntie" by everybody, even those who are not related to her (the sole exception is Captain Darling, who instead calls her "ma'am"). She and Bob often team up to try and defeat Russell.
• Captain Darling (voiced by James Kee)^[4] is Auntie First's beloved and loyal assistant. Darling will do anything to help Auntie, even if it means teaming up with her enemies, like Russell. He's in charge of the Near Under's By The Book Brigade. He's a very polite man, always apologizing to somebody when jailing them, and even when expelling people from Near Under. Speaks with an English accent. Unlike the rest of the cast, he calls Auntie First "ma'am". He wears a British police uniform and pink fuzzy slippers.

Other rulers

• Cousin Tess (voiced by Alison Sealy-Smith)^[4] — Ruler of Undersea (capital city: Bubble Town). She and Auntie First used to wage war on each other because they wore the same dress to a formal dance when they were in Despot School. Captain Darling put an end to the fighting by colorizing an old home video of the two fighting, deceptively coloring the dresses in different colors. Regardless, Auntie and Tess still fight all the time as a hobby, and as Populah puts it, "they enjoy hating each other".
• Hugh the Yu-Yu (voiced by Maurice Dean Wint)^[4] — Ruler of Under Under. He rules over the Yu-Yus (even though he is not one himself) and Slythers (including his pet Slyther, Mavis). He is very hippie-like, loves to party, and goes where life takes him, rolling a pair of fuzzy dice to make his decisions (even if it means disaster).
• "The Queen" — Ruler of Throng (on the continent of Ahead of Under). Like her people, the Queen (whose name has not been revealed in the show) has a phobia of germs, uncleanliness, and pretty much everything else. Bob worked for her in one episode, reversing her continent's direction to cure their motion sickness, by facing their sofas the other way, making it nearly flatten Under and causing Russell to intervene.
• King Lugobrius Rex — Ruler of the Empire of Flax. He summons many entertainers to his abode, and if they fail to please him, he then has them shaved naked and thrown in his dungeons forever. So far only Bob Wire's act, in which he inadvertently puts himself through large amounts of pain, has made Lugobrius laugh. He only speaks in wet burps, and is surrounded by massive Gordilators who do his bidding.

Anthropomorphic creatures

There are a variety of creatures who are quite intelligent and can be considered the "people" of Under (the world). Some species are common while others are rare to the point that only one or two members have been seen on the show, most of these rare creatures aren't listed here. Many species haven't been given canonical names and the majority of this section is conjectural.

• Underites - (various species) The inhabitants of the 'Kingdom' of Under (as opposed to the continent of Under or the entire Planet of Under, which plays host to various nationalities). Some people inhabiting certain areas of Under don't think of themselves as Underites, but as Florians, Outties, etc.
• Near Underites - (various species) The inhabitants of the Kingdom of Near Under, unlike Underites, the majority of Near Underites seem to be members of the Bulb-Headed Species).
• Throngegerians - (various species) The inhabitants of Throng, the town on Ahead of Under. Known for their cleanliness and of being afraid of almost everything, especially germs.
• Loopy's Species - One of the more common species (particularly in Under), with other notable members being Captain Darling, Mr. Queep, and Hugh the Yu-Yu. This species is characterized by high foreheads, large bulbous noses, and protruding jowls.
• "Populah's Species" - As juveniles (such as ex-Princess Populah), members of this species have no discernible noses, growing them later in life (such as Auntie First). Because this species is identical to Loopy's Species in adulthood, it is unknown how abundant or how rare it is in comparison. Because of the similarities, it could even be possible that both people are subspecies of one single race. Certain members of this species can breathe underwater (Populah, Cousin Tess), but it sometimes 'skips' a generation (as it did with Auntie First).
• Wire Guys - Sentient bundles of barbed wire taking a vaguely humanoid appearance (eyeballs on stalks extending up from the head, tails, two arms, and two legs). Like cyborgs, Wire Guys are very resilient and can use their coiled-wire form to extends limbs, filter through tight spaces, or even be totally unwound. Bob Wire is the only Wire Guy seen in the show, although a photo of his mother (a barbed wire fence) has been seen. This could mean that once Wire Guys 'die' they become normal inanimate wire. Bob also refers to himself as 'the Wire Man'. Being made of metal as well, he's also immune to organic ailments.
• Frags^[2] - Small green creatures with two arms and two legs, small tails, and small shark-like dorsal fins on the backs of their heads. Bob Wire's henchmen are both Frags (although there were originally three), and aren't called anything other than "Frags". They usually have high nasal voices, but sometimes their voices go low to emphasize certain words in their sentences.
• Sock boys - A race of cowboys that inhabit the wild-western Argyle Flats. They're woven sentient socks.
• Bulb-Headed Species - A common race very similar to Loopy's Species, but whose foreheads are shaped like lightbulbs (Loopy's Species have flat foreheads that taper off to a rounded dome). They reside mostly in Near Under.
• Yu-Yus - Small creatures (about the size of frags) that inhabit Under Under. They are usually white (somewhat resembling living marshmallows) with spiky hair that can't be discerned from the rest of their bodies. At least one blue Yu-Yu has been depicted in the show. Like their leader Hugh the Yu-Yu, actual Yu-Yus love to party and tame giant Slythers, earning themselves a reputation of being fierce and terrifying party animals.
• Guardilators - Large hulking people in the service of King Lugobrius Rex. They are tall with very muscular torsos and arms, and they have no noses.
• Florians - Plant people. Two notable factions, the Red Florians and the Yellow Florians had been waging a never-ending war over the last Thussle Bush. They communicate through smells and bursts of pollen.
• Cyborgs - Robots such as Vernon, who was built by the Clockmaker. The Hermit who lives on the moon may also be a cyborg since he appears to only be an upper torso with garbage can making up his lower portions and the usage of a single small wheel for locomotion. However, unlike Vernon, whose legs (also terminating in wheels) and arms can grow, the Clockmaker acts much more human (except his mode of transportation). Vernon can also extend his glasses to use them as microscopes or binoculars. Sometimes when his glasses are removed he is shown to not possess eyes behind them, while other times the eyes are very small. Because they are inorganic, they are unaffected by organic ailments and the Horrorborealis.
• Outties - The inhabitants of Outieville, they all have "outie" belly buttons.
• Ipples - The inhabitants of a small and secluded jungle village. They are unable to make decisions for themselves, preferring to ask either Loopy ("The Great Decider" of Ipple lore) and later a butterflier (on Loopy's instructions).
• Juice Beasts- Sentient creatures who can squirt juice out of their tentacles. One of the most popular locations in Under is the Juice Beast Stand, which serves as a major social gathering place (like a pub). The second most popular place is the Wriggly Pie Stand.
• Nossixs - Creatures that can change between being very small and weak to very big and muscle-bound while wearing wear lederhosens. Great-great Uncle Zed once defeated a Nossix and locked it away in a bongo drum until Bob Wire used the Nossix to chase Russell out of the kingdom.
• Ghouls - Any creature (sentient and non-sentient) during the Horrorborealis. They are zombie-like creatures that have an uncontrollable craving for pickles. They must be kept away from pickles until the Horrorborealis is over because if they eat them, they will remain Ghouls permanently. Being made of metal or having an invincible electro aura can keep you safe from glorification.

Miscellaneous creatures

• Sky-cow – A flying cow-like creature. Russell and his friends use these as transportation regularly.
• Squirt Hog aka "Engene" - An elephant creature that filled with water and a trunk nozzle to squirt. They're usually used for watering eggs to grow into buildings and other construction structures.
• Greenback – A green frog-like creature that was once used as currency when Russell attempted to introduce money to Under. When Bob took control of all the greenbacks, making Under destitute, Russell repealed money, which made the greenbacks worthless.
• Sky-beast – A more commonly used creature used as transportation by Bob Wire and Auntie First. They have handlebar-like horns and saddles which are generally used for this purpose by their riders.
• Slyther^[2] – A large raptor-like creature with a shovel-like mouth used for digging. Many are domesticated, ridden as transport, and milked to make Slyther Cheese by the Yu-Yus.
• Night-slyther – Although it sounds like a relative to the common Slyther, it is more monstrous-looking than the more common Slyther. It looks like a big green ball with multiple eyes, tentacles, long arms with claws and mouths on them, as well as a big spiky mouth on its body. They use to living in the shadows of Under Under hence would only emerge overground at night.
• Corola Vines - Similar looking to Night-Slythers but are carnivorous plants. Native to the Flatulent Swamps. On Pollen Holiday, their "hand-vines" grow excessively outward of the swamp to catch prey but are eaten by a migration of giant sea herbivores. Auntie First grows one in her greenhouse.
• Thoosh – A large walrus-like creature native to Wall-Eye Lake. It spouts jet-water to instantly grow Tither Ferns and eat them. Its water can also instantly clean places.
• Asymtote – A large monster that normally harmless. When hungry, every step it takes is half the length of its previous step before it (to conserve energy). It feeds on seaweed. It is also called 'A Monster that Never Arrives'. It also seems to be constantly humming a tone-deaf tune that induces high stress.
• Rebuloc – A vicious badger-like creature native to the Flatulent Swamp. Avoid.
• Blue Bird of Happenstance – A large blue bird with beak and eyes in reversed position (beak above eyes). Whoever it poops on is cursed with bad luck for a year. Its feathers bring good luck.
• Krenit – The ultimate defensive beast. It has a large body, multiple eyes to spot prey, and a super-fast sticky tongue that grabs anything it touches. You'll need slow plums to outrun a krenit, otherwise, you must stay absolutely still to avoid its tongue.
• Ippy Clippy – The ultimate offensive beast. It is the natural enemy of the Krenit. It has eyes on stalks, a large club-like tail, and two huge Popeye-like arms. It was so dangerous that they were run out of the continent. Great-great-uncle Zeb beat the last Ippy Clippy of Under and hid it inside the Wall Eye. Bob Wire rehydrated it to stomp Near Under's Krenit but it was beaten up by two Krenits.
• Slorch – A large red furry creature that can breathe fire. They only eat spicy hot peppers (from its home, the Hot Pepper Jungle) and need to live hot climates. They can also talk.
• Plurn – A large one-eyed squid-like fish. Baby Plurns live on land and breathe air, then the mother squirts them with ink and they turn into aquatic adults. The Plurn ink can also turn other land animals into fish-like creatures. Later in life, Plurns swim into the underwater Volcano Metamorpho and are erupted with wings, living the rest of their lives in the sky. Going into the volcano will return Plurned (ink-squirted) creatures back to their normal terrestrial selves.
• Spitesucker – A red leech-like creature. It feeds on anger. Once it is attached to someone, the only way to get it off is for the person to get so angry that the spitesucker flies off because of overload.
• Dream Gulper – A strange gelatinous creature. It can make dreams and nightmares real. If reality isn't corrected by sundown it becomes permanent.
• Gorble Birds – Hideous blue-birds with udders. They poop out gross smelly organic pink slop. They usually migrate to Near Under.
• Sea Clutch – A small fish-like creature. It got its name because it grabs things in its clutches.
• Rock Monkey - A monkey-like creature. Its arms are where its nose is supposed to be. Its favorite food is pumpernickel.
• Sea Sloth - A sloth-like creature native to the sea.
• Jelly Droppers - Large hovering jellyfish-like creatures. They drop huge balls of blue jelly. Cousin Tess uses them as heavy assault bombers.
• Skiffer Skink - A bloodhound-like creature with a pig's snout. Once it locks on a certain scent, it will never give up on the target.
• Great Big Hairy Eye-Ball - A floating hairy eye that temporarily turns anyone who looks directly at it into stone. Its only weakness is being squirted in the eye with the juice of SourSquirt Berries.
• Bedrock Beast - A large, stout caterpillar-like creature that eats any kind of rock or rock-derived material, such as brick houses.
• Parabat - A flying manta ray-like creature. It is used as a parasail on Frolicking Island.
• Sharkrunner - A red dinosaur-like creature with a shark-like head and a shark fin. It is used to pull the Parabat on Frolicking Island.
• Squump - A dumb, lazy cow-like creature. It gives both eggs and milk which are used for certain food products such as wriggly pies. When they get old, they leave to go spend their golden years in retirement. When that happens Slaphappies show up and cause trouble. The only way to get rid of Slaphappies is to get a new herd of squumps. To get a new herd you have to wrangle the herd leader which is usually the biggest and meanest Bull Squump. Bull Squumps have bull horns.
• Fang Ferret - A small ferret like creature with sharp fangs and a strong bite. Used at Miss Atilla's school for part of dispute battles between students.
• Donkey Horse - A donkey-like horse. They are treated like real horses in Argyle Flats.
• Gigagut - A giant stomach monster. It eats almost everything. It can't swim according to Populah since she said it would go to Near Under, then Under, then Under Under. This also reveals it can burrow. Consuming Vegidoubles of Quackvain Bushes makes it belches everything it had consumed.
• Slaphappy - An incessantly happy creature who loves playing pranks on townsfolk. Their natural enemy are Squumps since they're the only thing that will get rid of them. They come in four colors; red, orange, blue, and green.
• Pool Clam - A clam-like creature with three tongues that look like hands. They are quite common on Frolicking Island.
• Piranha Moth - A savage moth-like creature that eats clothing. It can eat people's clothes to their underwear.
• Skitchybug - A tiny blue bee like creature that swarms around people as pests.
• Brainsucker - A moving lump of flesh. They jump on people's heads and make them dumb as squumps. Loopy is curiously immune to this. Brainsuckers are afraid of light and only come out of their nest during the month-long eclipse.
• Ice Eel - Small flying eels that inhabit the Equatorial Glaciers, moving through the cracks in the ice.
• Pudrick - A small purple monster that acts as a "blame sponge". Unless blamed continually for everything that goes wrong, it will transform into a gargantuan creature and run amok.
• Fhrobish - Small, weasel-like creature. Its bite can cause Monsteritis.
• Fhrobish victims - Underites and humans that contracted Monsteritis from a Fhrobish, having changed into monsters.
• Dongo Worm - A giant worm that lives in the Uncertain City in Near Under. Anyone with Monsteritis needs to eat some of its mouth slime to revert to normal.
• Pilf - Tiny creature that looks vaguely like a rat with a robber's mask on. Fond of stealing anything and everything they can get their hands on.
• Lurber - Enormous plant creature with a highly specific diet: it only eats Kings. Its fronds (referred to as a "head-lawn") must be kept trimmed to keep the creature dormant.
• Guardgoyle - Vicious bulldog-like creature used as a royal guardian. It will obey the King of Under, whoever is wearing the crown that is.
• Hammer Rhino - Large animals that can be ridden and/or used as Battering Rams.
• Omnivorous Fatloid - A large obese gluttonous creature that inhabits the new garbage dump of Under, Glass Box Canyon. Since they are omnivorous, they will eat anything, including garbage and people.
• Dandilinium Vartibrus - A rare species of butterfly found in Under.
• Noflier - Another rare species of butterfly in Under. Sought by Mr. Queep in Ipple territory.
• Sea Scarf - Large aquatic creatures that migrate through the Edible Archipelago. They have sweet teeth and while they will eat Dessert Islands they will leave Salad Islands be. They also don't like marzipan.
• The Great Big Gehh - A mountain-sized snakelike creature that inhabits the Valley of the Outies.
• Shine Toad - In infant stage, they look like glowing frogs before metamorphosis. Later, transforms into large green feathery beings.
• Doubler - A mass of sentient green slime that can morph into any creature it wants to, even going so far as to copy a person's voice and mannerisms.
• Geet - A Giant creature that vaguely resemble a land-dwelling kraken. The adult Geet will use its giant tongue to catch food, ranging from people to cheese, pulling them back to its giant nest where the prey gets attached to the ceiling. The baby Geets then hatch and spit digestive enzymes onto the food until they liquefy and are then drunk by the babies.
• Gurd - A large swimming and walking creature that will eat people and then lay them as eggs. When the people hatch, they get imprinted and obsessed over the first thing they see.
• Dart Bird - Crane-like birds that are fired from crossbows, carrying letters in their claws. They travel great distances as Under's main courier service, even to Up.
• Monster Skiddadle Spray - Rabbits that spray repentant at monsters to chase them away. But they don't work on Bedrock Beasts though.
• Esculizard - Alligators with stairs in their mouths that revolve like escalators. Seen in the Big Cloud Mall.
• Squirt Pigs - Small pigs filled with paint to colorize anything. They can both squirt and mist color out like a spray can and hose.
• Excavator Camel- An all-terrain beast that Populah once used to help bring a Slorch back in its home.
• Hole Beast - A creature with a portal hole on its chest. Jumping through the holes will transport him/her to anywhere randomly, to another hole beast or distant place. You never know.
• Gob Shooter - A monster used to dirty up Under in one of Bob's schemes.
• Triviper - A three-headed snake that was used by Aunti First and Captain Darling to try and capture a giant worm. It wraps its tail around its target and snaps at its face.
• Spitballer - A monster used to dirty up Under in one of Bob's schemes.
• Foot Launcher - A monster used to dirty up Under in one of Bob's schemes.
• Megaphone Speaker - A megaphone creature whose mouth moves to whatever is being said or broadcast with it.
• Living Furniture - Living chairs, tables, ottomans, stools, scales, lamps, and many other things. Primarily seen in Under's castle. Some furniture isn't alive, such as Throne and Auntie First's desk.