[[folder:Main Students]]
!'''Adam Lyon'''
->Voiced by: Creator/NikaFutterman

A human boy who was transferred from Chester Arthur Middle School to Charles Darwin Middle School by an accidental misspelling of his last name from "Lyon" to "Lion".
* BodyHorror: At the end of "Hyena and the Mighty", Adam gets beat up by random machines after threatening Rufus to stop crying and Adam's beating makes Rufus laugh and results in Adam getting injured and bruised. But he deserved it for the massive jerk he was being.
* ButtMonkey: He constantly gets in trouble for Jake's misbehavior, gets hurt a lot, has bad luck, etc.
* TheChewToy: Most of his haplessness is played for comedy.
* DeadpanSnarker: Pays to be this if you're the OnlySaneMan of this school.
* FailureHero: Most of the time.
* TheHero: The closest this show has to one.
* JerkassBall: "The Hyena And The Mighty".
* JumpingOffTheSlipperySlope: "The Hyena And The Mighty"; his abuse of Rufus Hyena. Particularly the scene where he threatens the hyena with physical violence to stop crying.
* KickTheSonOfABitch: See BodyHorror above.
* TheNapoleon: He's in middle school and is short and stout.
* OnlySaneMan: ''Literally,'' and figuratively.
* RealMenWearPink: He can be rather effeminate at times and has been noted to have a rather girly voice.
* SensitiveGuyAndManlyMan: Sensitive Guy to Jake's Manly Man.
* StrangerInAStrangeSchool: A human boy accidentally sent to an animal school.
* TokenHuman: The only human of Charles Darwin Middle School.
* YouthfulFreckles

!'''Jake Spidermonkey'''
->Voiced by: Creator/TomKenny

A spider monkey who quickly became Adam's best friend. He is often very possessive of Adam and hates not having him around regularly. Jake is emotional and goofy who takes to heart some of Adam's comments and actions, leading to some overly dramatic and exaggerated reactions. He can be lazy and selfish, but at the same time he is also fairly sensible.
* AmbiguouslyGay: Jake appears to like Adam as a bit more than just his best friend. He is shown to be excited when he gets rainbows on his report card, plus he's obsessed with his own butt.
* CrazyJealousGuy: A rare male-on-male example whenever Adam hangs out with somebody else.
* EverythingsBetterWithMonkeys: A spider monkey.
* IdiotHoudini: Usually when he causes mischief, Adam gets the blame.
* {{Jerkass}}: Not such a good pal to Adam.
* JerkWithAHeartOfGold: He does care about Adam deep down.
* TheLancer: To Adam.
* LargeHam: Like other characters played by Creator/TomKenny.
* NeverMyFault: Has a tendency to blame others, mostly Adam, for his own troublemaking.
* PrehensileTail: Well he is a spidermonkey after all.
* RunningGag: His obsession with his own butt.
* SanitySlippage: Jake's mental state is incredibly fragile and can oscillate from calm and friendly to psychotically dangerous at a moment's notice.
* SensitiveGuyAndManlyMan: Manly Man to Adam's Sensitive Guy.
* WithFriendsLikeThese: He's willing to throw Adam under the bus whenever Jake does something bad.

!'''Windsor Gorilla'''
->Voiced by: Rick Gomez

A highly intelligent lowland gorilla who serves as the anti-bully at the school and often gives confusing wisdom to people. He speaks in a soft-spoken and nearly monotonous manner, often stringing together technical words as a matter of his normal speech.
* AmbiguouslyGay: In The Hyena And The Mighty, he calls Rufus Hyena his sweet, implying the two may be in a relationship despite Rufus being a one shot character. However, later, he was seen being attracted to another female gorilla.
* TheBigGuy: A large gorilla.
* DeadpanSnarker: Prone to sarcasm.
* GeniusBruiser: He's both intelligent and powerful.
* GentleGiant: Despite his massive physique, he's perfectly capable of taking care of a baby for a school project.

!'''Ingrid Giraffe'''
->Voiced by: Creator/GreyDeLisle

A female masai giraffe. Her towering height only adds to her feelings of insecurity as she tries desperately to blend into the background. She has a crush on Adam.
* {{Adorkable}}: Shows shades of dorky behavior. She's gawky, gangly and sweet.
* BreakTheCutie: Quite a few episodes do this to her.
* TheChick
* CuteAndPsycho: Her obsession with Adam causes her to become aggression and dangerously insane at times.
* CuteClumsyGirl: Tends to trip frequently.
* ClingyJealousGirl: She tries to break up Kerry and Adam more than once.
* DoggedNiceGirl: To Adam.
* HugeSchoolgirl: The tallest character in the show (her head is out of the screen most of the time).
* ShrinkingViolet: A shy gorilla.
* StalkerWithACrush: To Adam of all people.
* {{Yandere}}: She can get pretty crazy about her crush on Adam.

!'''Slips Python'''
->Voiced by: Rick Gomez

An easy-going, street-talking green tree python. He is an underachiever and excessive daydreamer who's gullibility allows him to believe anything he hears, which makes him a constant source of misinformation.
* BewareTheSillyOnes: Despite being a ditzy and a self-admitted "loser flunkie", he's the only member of the group who is an actual carnivore, and can be rather dangerous when he becomes hungry.
* CloudCuckooLander: His antics and personality make him border one being one
* TheDitz: Moreso the anyone else in the show. He is very gullible.
* TheHeart
* ReptilesAreAbhorrent: Averted. He's actually a pretty decent guy.
* SurferDude: Talks like this.
* TheStoner: Acts like one.

!'''Guadalupe "Lupe" Toucan'''
->Voiced by: Creator/GreyDeLisle

A female Keel-billed toucan, who is loud-mouthed, acerbic, dominating and materialistic, although she considers herself to be mild-tempered and low maintenance.
* JerkWithAHeartOfGold: Lupe has a good heart despite how annoying she is.
* GenkiGirl: Loud, exuberant, crazy at times.
* SpicyLatina: She talks with a Latino accent that's super seductive.

[[folder:Other Students]]
!'''Virgil "Bull" Sharkowski'''
->Voiced by: Creator/PhilLaMarr

A bull shark and the school's bully.
* AlwaysSomeoneBetter: His older sister is a way better bully than he'll ever be. He's extremely sensitive about this fact. Especially so when she managed to teach Adam to go frenzy in minutes though trolling. A concept he himself spent the whole episode trying to teach the ever oblivious Adam to no avail.
* BigLittleBrother: His older sister is much smaller he is.
* TheBully: At least until his big sister comes in.
* MobileFishbowl: Wears a special headset over his gills to breathe while on dry land.
* OnlyKnownByTheirNickname: He's mostly called "Bull"
* VerbalTic: Takes a deep breath between his words every once in a while.

!'''Henry Armadillo'''
->Voiced by: Creator/TomKenny

A nine-banded armadillo with a Texas accent and a reporter in the school media.
* UsefulNotes/AmericanAccents: Southern
* NerdGlasses
* TooKinkyToTorture: He seems to ''like'' everyone treating him like crap.

!'''Phineas Porpoise'''
->Voiced by: Creator/PhilLaMarr

A nerdy bottlenose dolphin.
* HollywoodNerd
* InsufferableGenius: Phineas has quite the large opinion of his intellect.
* TheLeader: Of his fellow nerds: [[ShownTheirWork a pig, an elephant, an octopus, and a parrot]].
* NonindicativeName: Though Phineas Dolphin doesn't quite have the same roll off the tongue.

!'''Margaret Rhino, Joanna Buffalo, and [=LeTanya=] Hippo'''
A trio consisting of a Sumatran rhino, cape buffalo, and hippopotamus.
* BigBeautifulWoman and BrainlessBeauty
* BigEater: All three.
* ValleyGirl: Joanna
* SassyBlackGirl: [=LeTanya=]

!'''James Ant'''
A red ant.

!'''Dickie Sugarjumper'''
->Voiced by: Creator/Tom Kenny

A sugar glider and the school jokester.

!'''Endugu Elephant'''
A Jamaican-American African Elephant.

[[folder:School Staff]]
!'''Principal Poncherello Pegone Pixiefrog'''
->Voiced by: Creator/MauriceLaMarche

The pint-sized African Bullfrog head of Charles Darwin Middle School.
* BunnyEarsLawyer: Although not very bright, he has been shown to handle crisis quite well.
* DeanBitterman: Averted. Though on one occasion he threaten Adam to get Henry to stay in the school[[note]]meaning a nausea-making student would return[[/note]] or else would send him to Siberia, though later he admit he was bluffing.
* PintSizedPowerhouse: He once beat a rhinoceros in a wrestling match.
* PetTheDog: When the entire student body (even Adam) turns on Slips because they don't understand that he's shedding, Pixiefrog clears up the misunderstanding by having them watch a documentary on shedding.
* TheNapoleon: He's a tiny frog that's in charge of the whole school.
* SmallNameBigEgo: And small body.
* WhyDidIthaveToBeSnakes: Well,...lawyers actually,

!'''Mrs. Geraldine Sharon Warthog'''
->Voiced by: Creator/GreyDeLisle

Principal Pixiefrog's secretary.
* ApatheticTeacher: Or secretary in this case.
* SassySecretary: Responsible for most of the jokes around Pixiefrog.
* TrademarkFavoriteFood: Chocolate, which she'll devour upon command.

!'''Mr. Cyrus Hornbill'''
->Voiced by: Creator/MauriceLaMarche

A rhinoceros teacher usually seen teaching whatever class is not covered by the other teachers.
* ButtMonkey: Mr. Hornbill has many misfortunes.
* GentleGiant: He's a rhino who's a good guy.
* NonIndicativeName: Hornbills are a group of birds found in Asia, but he's actually a rhinoceros.

!'''Mr. Maurice Mandrill'''
->Voiced by: MauriceLaMarche

The school counselor.
* [[HippieTeacher Hippie Counselor]]: He's the school's psychologist and music teacher..

!'''Miss Chameleon'''
->Voiced by: Nika Futterman

The overly-dramatic drama teacher.
* LargeHam: Her voice can go eerily high.

!'''Coach Tiffany Gills'''
-> Voiced by: Brian Doyle-Murray

A deep-voiced goldfish and the school's gym teacher.
* LargeHam: She almost always talks in a loud voice.
* MobileFishbowl: Teaches from a bowl wheeled around by Vice Coach Horace.
* TheNapoleon: She's a tiny goldfish who is aggressive.
* VocalDissonance: You wouldn't expect a goldfish to have such a deep voice, would you?

!'''Vice Coach Horace Ferret'''
->Voiced by: Creator/TomKenny and Robert Goulet in Animal School Musical

Coach Gills' ferret assistant.
* CuteMute: A cute ferret who doesn't speak.
* PerpetualFrowner: Not so smiley.
* SatelliteCharacter: To Coach Gills
* SilentSnarker: His facial expressions convey snark.
* TheVoiceless: Except in Animal School Musical
* WeaselMascot: More like Ferret Mascot.

!'''Mrs. Eugenia Tusk'''
->Voiced by: Creator/CreeSummer

The lunchlady.
* GentleGiant: A tall, kind sort.
* LoveInterest: To Mr. Hornbill (despite the fact she's married)

!'''Nurse Jacquline Gazelle'''
->Voiced by: Creator/GreyDeLisle

The school nurse.
* DumbBlonde: She mentioned that she's got a brain the size of a peanut in "Inoculation Day".
* JerkassBall: Tried to shave Rufus's hair off for no reason in "The Hyena And The Mighty".
* TheMedic: Due to being a school nurse.
* SchoolNurse: Duh.

!'''Mr. Blowhole'''
->Voiced by: Creator/DeeBradleyBaker

A whale who is usually seen as the teacher of the nerds. Accompanied by whale noises whenever he talks.

!'''Kerry Anderson'''
->Voiced by: Creator/CreeSummer
* AmbiguouslyBrown
* {{Love Interest|s}}: She's one of Adam.
* PeekABangs