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.
- Always Someone Better: To Baboon, who is always trying to upstage him, unsuccessfully.
- Badass Pacifist: Early on, he deals with enemies peacefully (he considers tripping a rampaging Baboon with his foot to be distasteful). Over time, he becomes more willing to hurt people—though they usually have it coming.
- 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."
- Fate Worse than Death: The Princess in the Cinderella parody. Marrying her was something Weasel considered to be the trope.
- Gentleman and a Scholar: A polite and selfless genius, in contrast to Baboon's selfish Insufferable Imbecile. While Baboon sees him as an Insufferable Genius, he is far from being one. In fact, he's typically very modest.
- Humble Hero: He tells people not to worship the ground he walks on for his benevolent acts to little effect.
- 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.
- 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".
- Loved by All: An extremely beloved public figure who goes out of his way to help everyone with his genius intelligence.
- 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.
- Renaissance Man: Art, cuisine, law, medicine, science, sports... name it, Weasel's a foremost expert on it.
- Surrounded by Idiots: As the seasons progress he becomes a more pretentious Insufferable Genius 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.
- Vocal Dissonance: Short, shrimpy Weasel speaks with the dulcet, sophisticated tones of Michael Dorn.
- 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.
- Bare-Bottomed Monkey: His defining character trait aside from being an idiot is his big bright red behind.
- 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.
- A Day in the Limelight: He's the playable character of the Cartoon Network game The Envelopes, Please!
- 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.
- Heel–Face Revolving Door: Sometimes he's Weasel's nemesis, sometimes they're allies. Starting with Season 3 he's more consistently portrayed as a vitriolic best bud to Weasel.
- Hulk Speak: We'll just illustrate with his first lines from the pilot 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.
- Jerk with a Heart of Gold: He is sometimes friends with I.M. Weasel.
- Joke of the Butt: His big red butt is milked intensely for humour.
- Kindhearted Simpleton; Usually when he isn't antagonizing Weasel or isn' tDrivenByEnvy to surpass Weasel, he is very nice and friendly, but he is still prone to mess things up out of idiocy.
- Know-Nothing Know-It-All: An even dumber version than usual for this trope, since he can't even use complete sentences, but is still absolutely convinced he knows everything, especially in early episodes. This of course invariably lead to disaster.
- Maniac Monkeys: Sometimes he's spiteful, stubborn and stupid enough to bring society crashing down or destroy the whole planet...
- Mischief-Making Monkey: ...other times he's just kind of a goofy troublemaker or even a borderline Kindhearted Simpleton.
- Naked People Are Funny: His big red butt.
- Psychopathic Manchild: At his worst, he's prone to destructive tantrums and is childishly petty.
- Reality Warper: One episode depicts him being able to warp reality with his monkey paws. He's not smart enough to use this ability more than once, however.
- Team Rocket Wins: There are rare times he gets some recognition and even comes out on top over Weasel.
- 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. He's convinced he's smarter and more capable than Weasel with nothing to back it up.
- Straw Loser: Baboon has very little going for him—he's clueless, irrational, spiteful, untalented and unlucky. Thus, Weasel typically comes out on top in their Comically Lopsided Rivalry.
- 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 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. For tropes relating to him on Cow and Chicken, see here.
- Creepy Crossdresser: In the "Robin Weasel" episode, where he takes Maid Marion's place, and both Weasel and Baboob are horrified to discover him imprisoned in the castle. And so is Scheriff Nottingham (played by Jolly Roger) who didn't kidnap her/him this time.
- 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.
- Joke of the Butt: Just as bum-focussed as he is in Cow and Chicken.
- Large Ham: He's as loud and melodramatic as he was on Cow and Chicken.
- Laughably Evil: He is usually mischievous if not outright malicious and he also brings a lot of humor to the show.
- 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. However, it is never made clear if it is real or fake.
- Rogues' Gallery Transplant: After a season's worth of episodes of just antagonizing Cow and Chicken, he began to harass Weasel and Baboon. He actually explained in his first appearance in an I Am Weasel short that he was attempting to hide from Supercow where she'd hopefully not look — in a Weasel cartoon. She beats him up anyway.
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.
- Interspecies Romance: The show constantly implies this, but never goes beyond that.
- Mayfly–December Romance: As is shown in "I Am My Lifetime", when Lullabelle still spends time with Weasel at the Retirement Home for Elderly Cartoon Heroes... after 4 years of knowing him since he was born, because that's about as long as weasels live. As such, he's in his old age, while she has barely aged at all.
- The Smurfette Principle: The only recurring female character unique to the show.
A Naval officer who occasionally asks Weasel for help.
- Ambiguously Brown: He had darker skin in his final appearance for reasons unknown.
- Chuck Cunningham Syndrome: Like Lullabelle, he stopped making appearances on the show after the third season.
A buffoonish man dressed as a sailor who filled in whatever roles were needed. Like Red Guy, he originally appeared on Cow And Chicken, and ironically got more roles on this show than in that one.
- Ascended Extra: Only appeared once on Cow And Chicken, but got all the way to minor character status here.
- The Artifact: His name, more or less. In his first appearance (in the Cow and Chicken episode "Pirate Lessons"), his name is a Pun; he's a jolly man named Roger that the Red Guy was using as his "jolly roger" (the flag for his pirate ship). His appearances in I Am Weasel don't take advantage of the pun.
- Fat Idiot: Competes with Baboon for the role as the dumbest character on the show.
- Narrator All Along: In "My Blue Hiney"
- Unreliable Narrator: Turns out to be the narrator for the episode "My Blue Hiney", which portrayed Weasel and Baboon as a classic country music duo, only for Weasel to interupt him at the end to point out that the whole story was total nonsense.
Cow and Chicken's loud, obnoxious teacher. Like The Red Guy and Jolly Roger, she too made the leap from Cow and Chicken to I Am Weasel in its later episodes.
- Ascended Extra: Not quite to the same extent as The Red Guy or Jolly Roger, but she appears more than once in I Am Weasel as her own character.
- A Day in the Limelight: "I.M.N. Love" is her largest role in either of David Feiss' shows.
- Gonky Femme: More pronounced here than in Cow and Chicken, since female characters on I Am Weasel tend to be quite attractive (see Lullabelle above).
- No Indoor Voice: As usual with her.
- The Smurfette Principle: More or less fills Lullabelle's niche as the only recurring female character.