The main protagonist, a heroic weasel, who is everything I. R. Baboon wishes, and possibly imagines, he was. Weasel is often seen volunteering to help those less fortunate than himself, or contributing great deals to society by erecting monuments and structures with beneficial results.
- The Ace: He's famous, brilliant, multitalented, and loved by everyone.
- Badass Pacifist: For the most part.
- Broken Ace: Later episodes start to show some dents in Weasel, where he often shares the Butt-Monkey role with Baboon.
- Butt-Monkey: Weasel would develop his fair share of bad luck in later episodes (thus toning down his "Mr. Perfect" character).
- Chronic Hero Syndrome: Will go out of his way to do the right thing in any situation."One life lost diminishes my own."
- Cinderella Circumstances: In the Cinderella parody.
- Fate Worse than Death: The Princess in the Cinderella parody. Marrying her was something Weasel considered to be the trope.
- Hypercompetent Sidekick: Weasel plays this role in episodes like "Baboon Man and Weasel Boy" or "I.R. Robin Hood" where he is the assistent to Baboon's Idiot Hero.
- I Am Not Weasel: The Trope Namer, sort of. Case in point: Red Guy mistook him for a squirrel.
- Insufferable Genius: He comes off as this from Baboon's perspective.
- Invincible Hero: More in earlier seasons.
- Large Ham: Less so than the Red Guy, but it's still noticeable at times. A notable case of where he goes full-on ham is in the episode, "I Am Franken-Weasel".
- Nice Guy: Calling him nice would be an understatement. He was first shown giving a dying boy a kidney transplant. He becomes slightly more egotistical and neurotic in later episodes, though considering how much his kindness often gets him, it's often justified.
- Only Sane Man: Often the show's universe appears to almost be equally as brainless as Baboon, with Weasel usually playing the Straight Man.
- Parody Sue: In early episodes, Weasel played The Ace rather genuinely, at rather unrealistic levels.
- Surrounded by Idiots: As the seasons progress he becomes a more pretentious Small Name, Big Ego that suffers due to living in a world of fools.
- Trademark Favorite Food: Eggs. Doubles as Truth in Television, as weasels love eggs. It's also revealed in "I.R. in Wrong Cartoon" that he loves pie, as well.
- Wholesome Crossdresser: Every once in a while. When a couple asks for replacement kids in the episode "Leave It to Weasel", he's the daughter. He also portrays Cinderella in the parody episode, "The Fairy Godfather", and while this isn't present in the episode itself, he wears a dress in the title card for "Back to School".
- Wicked Weasel: Inverted. Weasel is a courageous, selfless hero who is great at everything, pretty much the exact opposite of that trope's cowardly, self-centered sociopath that often has to resort to trickery and deceit to accomplish anything. This characterization of near-perfection serves primarily to set him up as the comedic Foil to I.R. Baboon.
The other main character of the series. Most times he is seen taking a back seat to Weasel's escapades, and feeling resentful and jealous towards him. But there are those rare occasions when the two act as the best of friends.
- Butt-Monkey: No pun intended. Baboon tends to suffer, though it's mostly his own fault.
- Chaotic Stupid: In the earlier seasons when he was the antagonist, many conflicts were the result of his idiotic and ludicrous actions.
- Deuteragonist: The main secondary focus.
- The Ditz: He's dim-witted and uses incorrect grammar when he speaks.
- Driven by Envy: Early on, Baboon (unsuccessfully) does anything he can to spite or outdo Weasel. This is toned down later, as he and Weasel become allies. Though I.R. will still try to supplant him if he has the chance (or thinks he does).
- Dumbass Has a Point: Once every so often, he manages to make a good point that even Weasel admits to.
- Half-Dressed Cartoon Animal: After all, he doesn't need pants for the victory dance.
- Happy Dance: He used to have a victory dance before it got phased out. It was used regularly in the opening titles.
- Hulk Speak: We'll just illustrate with an example from the first episode, where he resolves to complete a half-done bridge on his own:"But...bridge must finish! We can't wait 78 year! (gasp) Wait. Minute. What if I.R. finish bridge I-self?! I.R. get all credit! GLORY TO I.R.!"
- Hypocritical Humor: He frequently points out that the Red Guy isn't wearing any pants, when he goes pantsless himself.
- I Just Want to Be Special: If not also trying to one-up Weasel.
- If I Were a Rich Man: In the comic book story "X Marks the Baboon", I.R. Baboon finds a treasure map and imagines himself attending a fancy party and owning several diamonds and a rubber band collection.
- Insufferable Imbecile: He plays this as a foil to I.M. Weasel. Whereas Weasel is scholarly, uber-competent, and selfless, Baboon is a selfish underachiever who constantly tries upstaging Weasel, who, even knowing this, considers him a friend and even goes out of his way to save his bacon.
- Naked People Are Funny: His big red butt.
- Psychopathic Manchild: At his worst, he's prone to destructive tantrums and is childishly petty.
- The Resenter: The main drive of his character. He resents Weasel's fame and considers himself better, even though Weasel is a certified genius, whereas Baboon is... well, not.
- The Rival: To Weasel in the earlier seasons.
- Small Name, Big Ego: Baboon's brain is as small as his ego is large.
- Third-Person Person: He refers to himself in third-person.
- Too Dumb to Live: When he inevitably screws something up, Weasel has to bail him out.
- Trademark Favorite Food: Bananas. He's a baboon, what did you expect?
- Wicked Stepmother: Well, wicked stepbrother, but neither him nor Cinder-Weasel had any parents in the Cinderella parody episode, "The Fairy Godfather".
The Red Guy/I.B. Red Guy
The same character from Cow and Chicken (only he goes by I.B. Red Guy or the many names that refer to his not wearing pants), who acts as an antagonist of the series whenever I. R. Baboon isn't fulfilling the role.
- The Devil Is a Loser: If he's not running some aimless fraudulent business, Red Guy will sometimes be the damsel in distress Weasel and Baboon need to protect.
- Flaming Devil: Red resembles the devil and can be pretty flamboyant.
- Jerkass: He can vary from this to Jerk with a Heart of Gold depending on the episode.
- Large Ham: He's as loud and melodramatic as he was on Cow and Chicken.
- Otherworldly and Sexually Ambiguous: In spades. Considering he's a jack-of-all-trades, he can pull any kind of role of either gender... and not only that, whenever he plays a female, he clearly has a bust.
- Rogues-Gallery Transplant: After a season's worth of episodes of just antagonizing Cow and Chicken, he began to harass Weasel and Baboon.
The Weasel's blonde and attractive nurse assistant. A minor character, she made most of her appearances in Season 2 and went out of focus afterwards. She is often speculated to be Weasel's girlfriend.
- Big Eater: Lullabelle is shown to have a voracious appetite in "I.R. Ice Fisher".
- Chuck Cunningham Syndrome: She disappeared after her appearances became sporadic in the third season.
A Naval officer who occasionally asks Weasel for help.