A green monster under the bed covered in purple warts who likes having fun and going into all sorts of wild adventures.
- Cephalothorax: His head is also his body.
- Child Hater: It is established in "Baby Snail" and "Baby Jail" that he doesn't care for babies, though both circumstances have him change his tone, growing quite attached to the baby snail in the former and understanding that even infants have it rough in the latter.
- Gasshole: Is constantly burping after eating socks or any other items.
- Gonk: As a monster from the 90s, he prides himself on his grossness.
- Jerk with a Heart of Gold: He can be greedy and impulsive, but he does care about his friends Squishington and Molly Coddle and will always be there to help them (though he usually makes things worse before they get better). He also sincerely cares for the boy whom's bed he lives under.
- Large Ham: Voiced by Jim Cummings, it's to be expected.
- Reluctant Monster: When Mr. Bumpy hears about a monster that is terrifying his boy, his first instinct is to find the interloper and beat the snot out of him. Only later he finds out that his hard-partying antics are what's upsetting the kid, and he is the monster, much to his horror.
- Trademark Favorite Food: He loves eating socks.
A blue blob monster who lives in the toilet and prefers things to be clean.
- Camp Straight: He cares a great deal about cleanliness and the arts, but has occasionally shown romantic interest in the Cute Dolls and also ends up reciprocating the Germ Girl's affections toward him.
- Interspecies Romance: Ends up falling for the Germ Girl in "Adventures in Microbia".
- Neat Freak: He's quite obsessive when it comes to cleaning.
- Sudden Name Change: Downplayed. He is called Squishington throughout the series, but is given two different full names, both of which are contradictory on whether Squishington is his first name or his last name. He states that his full name is Squishington A. Peabody in "Sock It to Me", while "Love's Labor Bumped" has Mr. Bumpy refer to him in passing as F. Lee Squishington.
A comfort doll who likes to hang with Bumpy and Squishy.
- Beware the Nice Ones: Even a sweet comfort doll can be pushed too far, and when she is, it makes even Destructo nervous.
- Heel Realization: In "Twas the Night Before Bumpy", she's devastated when she realizes she was wrong by resorting to being a Prima Donna Director.
- Living Toys: She is a doll that happens to be alive.
- Nice Girl: She is a very warm and friendly person and it's in her nature to be comforting and helpful.
- Prima Donna Director: When no one takes her seriously after Bumpy appoints her pageant director in "Twas the Night Before Bumpy", she resorts to being demanding and petulant.
- Punny Name: Her name is a play on "mollycoddle", which means to be overprotective and excessively pampering toward someone.
The Cute Dolls
A clique of Barbie-esque dolls who tend to look down on the main cast.
- Alpha Bitch: All of them are dismissive and scornful of those they don't consider fashionable.
- Girl Posse: They're all very snooty and vainglorious.
- Living Toys: They're dolls that happen to be living beings.
A toy robot who goes to great lengths to enforcing the rules, which often puts him at odds with Mr. Bumpy.
- Abhorrent Admirer: To Molly, at one point.
- Big-Bad Ensemble: Alongside the Closet Monster, he is the closest the show has to a main antagonist.
- Do-Anything Robot: He has an awful lot of gadgets built into him even though he's a toy. He can even make milkshakes!
- Hero Antagonist: He cares about the rules, so he's technically a good guy, but it's the harsh methods in which he enforces the rules and his constantly being at odds with Bumpy and friends that make him an antagonist.
- Living Toys: He's a living toy robot.
- Villain Song: He sings "I'm Bigger Than You" in one of the Karaoke Cafe segments, where he boasts about using his size and power to get everyone to do what he says.
A monster made of dirty clothes who often menaces the main characters.
- Big-Bad Ensemble: Along with Destructo, the Closet Monster is the closest the show has to a main antagonist.
- The Dreaded: Practically everyone in the house fears him.
- The Voiceless: He doesn't talk, but he does do a lot of growling.
Sleemoth and Gloog
A pair of dim-witted aliens who occasionally try to invade Earth, only for Bumpy and friends to ensure they suffer a humiliating defeat.
- Alien Invasion: They're aliens that want to conquer Earth.
- Aliens of London: In spite of not being from Earth, Sleemoth has a Scottish accent and Gloog has a British accent.
- Extra Eyes: They both literally have four eyes.
- Fat and Skinny: Sleemoth is skinny, while Gloog is fat.
- My Brain Is Big: Sleemoth's brain is larger than Gloog's and he's the more intelligent of the pair. He even attempts to enlarge his brain to become even smarter in "Comfort Schmumfort". In fact, his brain becomes so huge that he actually starts physically bludgeoning Molly with it.
- Squishington: "Good Golly! Miss Molly is being crushed under the sheer weight of his intellect!"
- Politically Incorrect Villain: In the episode "Comfort Schmumfort", they dismiss Molly Coddle as not being a threat just because she's a girl.
- Small Name, Big Ego: They are quite arrogant and always assume that their plans can't be thwarted.
- Those Two Bad Guys: They're never seen apart.
- Villain Song: In the episode "It Sang from Beyond the Stars", they pretend to be a rock band and sing a song called "Invaders", which explicitly mentions their plans for invasion in the lyrics.
A germ girl who ends up becoming a love interest for Squishington.
- Abhorrent Admirer: Squishington initially doesn't want anything to do with her since he's obsessed with cleanliness and she's a germ, but he eventually reciprocates her feelings.
- Of course, then she infects his body after he's restored to his normal size, getting him sick.
- Extra Eyes: She has five eyes.
- No Name Given: She's never mentioned by name.
- Non-Mammal Mammaries: Has some prominent breasts despite being a bacterium.
- Pink Means Feminine: She wears pink clothes.
A tiny toy robot (A "Turbo Totranoid") from Japan who first encountered Mr. Bumpy and Squishington after Bumpy won her from mailing enough cereal box tops. While capable of becoming large and menacing, she isn't really that hostile.
- The Bus Came Back: Her debut episode ended with her going back to Japan, while she returned in "Made in Japan II" after the company that made her started recalling and destroying her sisters.
- Cute Machines: She's a tiny toy robot who's cute as a button.
- Hulking Out: Transforms into a larger form whenever she's frightened or needs to defend herself.
- Meaningful Name: True to her name, she's a robot who's rather small.
A conniving silverfish who first appeared in "I Dream of Silverfish", where he manipulated Mr. Bumpy and Squishington into being his personal slaves by pretending to be a genie.
- Ambiguously Jewish: In "'Twas the Night Before Bumpy", his musical number is a version of the Dreidel Song.
- Big Creepy-Crawlies: He's a silverfish who, while as small as the main characters, is still larger than normal.
- Cigar Chomper: He's depicted with a cigar in his debut episode.
- Half-Dressed Cartoon Animal: He wears a vest and a bowtie, but no pants.
- Species Surname: He's a silverfish who's last name is Silverfish.
Odiferous J. Stench
A stinkbug who becomes infatuated with Molly Coddle after she gets covered in garbage in the aptly-titled episode "Love Stinks".
- Abhorrent Admirer: After noticing how stinky Molly is when she's covered in garbage, he becomes an unwanted suitor toward her.
- Bi the Way: He starts out attracted to Molly Coddle before refocusing his affections toward Bumpy and later falling in love with the Closet Monster.
- Eyes Always Shut: He's never seen not squinting.
- Meaningful Name: He's a stinkbug and his given name is a synonym for smelly.
- Mysterious Middle Initial: It's never explained what the J stands for.
A baby Mr. Bumpy encounters in "Baby Jail".
- Large and in Charge: He's bigger than the other babies and is their leader.
Juaquin Gusanito Sin Manos
An earthworm Mr. Bumpy and Squishington meet in South America during their quest toward the North Pole in "'Twas the Night Before Bumpy". He wants a pair of robotic arms so he can easily do stuff requiring hands.
- Bilingual Bonus: "Gusanito Sin Manos" is Spanish for "worm without hands".
- Disney Death: He appears to die when one of the pillars of Stonehenge crushes him, but it's later revealed that he actually survived by ducking back into the earth before the pillar fell.
Doris D. Bird
A British hummingbird Mr. Bumpy and Squishington encounter at Stonehenge in "'Twas the Night Before Bumpy". She wishes for a jetpack so she wouldn't have to tire herself from flapping her wings constantly every time she had to travel.