!'''Adam Lyon'''
->Voiced by: Nika Futterman

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".

* AssholeVictim: In Hyena And The Mighty, after Adam is being [[OutOfCharacterMoment uncharacteristically mean to poor old Rufus,]] he gets injured by random machines in a stunt. But you can't feel bad for him for being a jerk to Rufus.
* 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
* TheChewToy
* DeadpanSnarker
* 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.''
* StrangerInAStrangeSchool: A human boy accidentally sent to an animal school.
* TokenHuman
* 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: Has some shades of this
* [[ClingyJealousGirl Clingy Jealous Guy]]
* EverythingsBetterWithMonkeys
* IdiotHoudini: Whenever he does something that causes trouble, Adam is always the one who gets blamed.
* TheLancer
* LargeHam

!'''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 gorrila
* TheBigGuy
* DeadpanSnarker
* GeniusBruiser: He's both intelligent and powerful.
* GentleGiant: Despite his massive physique, he's 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}}
* BreakTheCutie: Quite a few episodes do this to her.
* TheChick
* CuteClumsyGirl
* ClingyJealousGirl: She tries to break up Kerry and Adam more thann once.
* DoggedNiceGirl: To Adam.
* ShrinkingViolet
* StalkerWithACrush: To Adam of all people.
* {{Yandere}}

!'''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.

* CloudCuckooLander: His antics and personality make him border one being one
* TheHeart
* ReptilesAreAbhorrent: Averted.
* 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
* GenkiGirl
* SpicyLatina: She talks with a Latino accent that's super seductive.

!Other/Recurring Characters

!'''Virgil "Bull" Sharkowski'''
->Voiced by: Creator/PhilLaMarr

A bull shark and the school's bully.

* AlwaysSomeoneBetter: His big 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.
* 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"

!'''Rufus Hyena'''

* {{Adorkable}}
* AnnoyingLaugh
* BreakTheCutie: Insulted by Adam, cried several times throughout his episode, terrorized by Nurse Gazelle, threatened by Adam at one point
* ChuckCunninghamSyndrome Rufus never appears again after Hyena And The Mighty.
* DarkIsNotEvil: He's a spotted hyena but is easily one of the most sympathetic characters on the show besides Ingrid Giraffe.
* TheHyena
* TheVoiceless
* {{Keet}}
* MoodSwinger: So very much.
* OcularGushers: Does this a LOT.
* PuppyDogEyes: He's downright ADORABLE when he pulls it off.

!'''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.


* LadyLooksLikeADude: Although she more sounds than looks like, however, she does, for some particular reason, have a mustache
* LittleBigSister: She's older than Bull, yet he is bigger than she
* PinkIsFeminine: First seen wearing a pink outfit and her "head phone" are a hot pink
* TomboyishName: Euripides is a male name
* YourMom: Her insults feature this

!'''Deidre Koala'''

* BigEater: She loves anythinng eucalyptus, but takes a long time to digest it.
* TheScrappy: In-universe example.

!'''Larry Raccoon'''

* {{Delinquent}}

!'''Principal Poncherello Pegone Pixiefrog'''
->Voiced by: Creator/MauriceLaMarche

* 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 beats a rhinoceros in a wrestling match.
* TheNapoleon: He's a tiny frog that's in charge of the whole school.
* SmallNameBigEgo

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

* SassySecretary: Responsible for most of the jokes around Pixiefrog.
* TrademarkFavoriteFood: Chocolate, which she'll devour upon command.

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

* ButtMonkey
* GentleGiant

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

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

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

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

* LargeHam: Her voice can go eerily high.

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

* LargeHam: She almost always talks in a loud voice.
* MobileFishbowl: teaches from a bowl wheeled around by Vice Coach Horace.

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

* CuteMute
* PerpetualFrowner
* SilentSnarker
* TheVoiceless
* WeaselMascot

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

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

* DumbBlonde: She mentioned that she's got a brain the size of a peanut in "Inoculation Day".
* TheMedic
* SchoolNurse

!'''Ms. Gretchen Loon'''
->Voiced by: Creator/CreeSummer

!'''Ms. Bear'''
->Voiced by: Creator/TomKenny

!'''Kerry Anderson'''
->Voiced by: Creator/CreeSummer

* AmbiguouslyBrown
* {{Love Interest|s}}: She's one of Adam's and Chad's.
* PeekABangs

->Voiced by: Creator/TomKenny

!'''Possum Girl'''
->Voiced by: Dannah Feinglass

* StalkerWithACrush


* BigEater: Loves scarfing cakes, watermelons, and marshmallow mega-squares.