The eldest of the septuplets, One is a tomboy who often tries to be responsible, but is usually undone by her bossiness. Although she is easily worked up and gets frustrated by her siblings' antics, she really does care about them.
Voiced by Joanne Vannicola
- All Flyers Are Birds: Sort of; she not only has wings but also birdlike talons for feet.
- Cute Monster Girl
- Fiery Red Head
- Flight: She's the only monster who can fly.
- Nice Cap
- Pink Means Feminine: Subverted. Her cap, long shirt, and shorts are all shades of pink, but she's a tomboyish sports girl.
- The Stool Pigeon: Sometimes; most notably in the episode "I'm Telling".
- Team Mom: She tries to act like one when their mother isn't around, but she can't keep her rambunctious siblings under control at all, nor can she keep her patience with them.
- Tomboy and Girly Girl: With Six
- Winged Humanoid: Has a pair of short wings on her back.
The most helpful of the monsters, Two has a very prominent nose which he often sticks where it doesn't belong. Bit of a goody two shoes.
Voiced by Colin Mochrie
- Butt-Monkey: Often meets with misfortune either from his siblings or through humorous circumstances.
- Cursed with Awesome: He goes through an episode believing this until his noses leads him to find their missing pet.
- Gag Nose
- Nice Guy
- The Nose Knows
- The Resenter: Occasionally.
- Sitcom Character Archetypes: The Square.
- Straight Man: Shares this role with One.
- Strong Family Resemblance: Their mother often says that he reminds her a lot of his father.
The most dramatic monster who takes on a new role each episode. Although he is perhaps the weirdest monster, he also seems to have the most knowledge.
Voiced by Dwayne Hill
- Attention Whore
- Breaking the Fourth Wall: Does this occasionally.
- Cloud Cuckoo Lander
- Cosplay: In most episodes.
- Ditzy Genius / Genius Ditz
- Drama King: Quite literally. He takes on a different character role almost every episode.
- Large Ham
- Man of a Thousand Voices: His actor gives him a distinct voice for every persona.
- Mr. Exposition: He's usually the one to explain a new concept to the others.
- Why Did It Have To Be Mice?
Grumpy, selfish and rambunctious, Four has a troublesome and negative personality. Despite his flaws, though, he loves his siblings and usually does the right thing in the end. He is almost always seen with Five.
Voiced by Sean Cullen
- Agent Scully: He is by far the most skeptical of the monsters.
- Butt-Monkey / No Respect Guy: His Jerkass tendencies don't earn him any respect from his siblings.
- Deadpan Snarker
- Grumpy Bear
- Half-Dressed Cartoon Monster
- Heterosexual Life-Partners: With Five.
- Jerk with a Heart of Gold
- Middle Child Syndrome
- The Napoleon: He is the shortest of his siblings, although he's still a giant.
- Nice Hat
- Red Eyes, Take Warning: Downplayed. Not evil, just occasionally a jerk.
- Sibling Rivalry: With Three, with whom he gets into a fair amount of arguments. He also frequently challenges Three's knowledge.
- With One, because he wants to be in charge and resents having not been born first.
- Vocal Evolution: His voice was originally deeper and gruffer in the first two episodes, "Good Morning" and "Good Night".
- You've Gotta Have Green Fur
The most child-like of the monsters, Five follows Four like a lost puppy and seldom speaks, and when he does he speaks with a thick accent that requires Four to translate for him.
Voiced by Sean Cullen
- Acro Fatic: The biggest monster but no less rambunctious than the others.
- Big Eater
- Big Fun
- Cheerful Child
- Elmuh Fudd Syndwome: When he does talk.
- Extreme Omnivore
- Friend to All Living Things: He is especially fond of animals.
- Gag Nose
- Morality Pet: Four's.
- Overly Long Tongue
- The Unintelligible: Though it's a little more comprehensible, even in later episodes. Four translates for him.
- You've Gotta Have Blue Fur
The resident ballerina, Six thinks she's the most beautiful of the monsters, and often worries about her appearance.
Voiced by Michele Scarabelli
- Ballet: She goes to ballet class, and is always seen wearing her tutu and carrying her ballet wand.
- Beautiful Monster Girl
- The Ditz: She isn't really stupid at all, but she occasionally has airhead moments, mostly by being oblivious to her surroundings.
- Drama Queen
- The Fashionista
- Girly Girl
- Graceful Ladies Like Purple
- The Klutz: Ironically, for the most graceful monster, she is.
- Pride: In her appearance.
- Running Gag: For someone so graceful, she's tripping over her own feet every other episode.
- Sleepy Head: She needs her beauty sleep, after all.
- Tomboy and Girly Girl: With One.
The youngest of the septuplets, Seven is the tallest but most timid and gentle of the monsters. He has a detachable head, which he is prone to losing.
Voiced by Sean Cullen
- Cranium Chase: This happens to him occasionally, and is always Played for Laughs.
- The Klutz
- Losing Your Head
- Lovable Coward
- Nervous Wreck
- No Celebrities Were Harmed: His voice is likely an allusion to that of Boris Karloff, as his physical appearance calls creatures such as Frankenstein's Monster to mind somewhat.
- Perky Goth: He's got the voice and look of a classic movie monster, but is in reality very sweet.
- Required Secondary Powers: His head screws on and rolls around a lot when detached but he appears immune to motion sickness.
- The Storyteller
- Trademark Favorite Food: Cheese.
- Why Did It Have To Be Bugs?
- And he also has a weird fixation on hedgehogs.
The monsters' mother. A tiny woman who speaks in a heavy accent and usually provides her children with support and the episode's Aesop.
Voiced by Debra McGrath