'''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".

* 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
* JerkassBall: The Hyena And The Mighty. Big time.
* 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.
* TheNapoleon
* OnlySaneMan
* StrangerInAStrangeSchool: A human boy accidentally sent to an animal school.
* TokenHuman
* YouthfulFreckles

'''Jake Spidermonkey'''
->Voiced by: 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
* [[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: RobPaulsen

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.
* TheBigGuy
* DeadpanSnarker
* GeniusBruiser
* GentleGiant

'''Ingrid Giraffe'''
->Voiced by: 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
* DoggedNiceGirl
* ShrinkingViolet
* StalkerWithACrush: To Adam of all people.
* {{Yandere}}

'''Slips Python'''
->Voiced by: RobPaulsen

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.

* TheHeart
* SurferDude: Talks like this.
* TheStoner

'''Guadalupe "Lupe" Toucan'''
->Voiced by: 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

'''Virgil "Bull" Sharkowski'''
->Voiced by: PhilLaMarr

A bull shark and the school's bully.

* TheBully
* MobileFishbowl: Wears a special headset over his gills to breathe while on dry land.

'''Rufus Hyena'''

* {{Adorkable}}
* AnnoyingLaugh
* BreakTheCutie: Insulted by Adam, cried several times throughout his episode, terrorized by Nurse Gazelle, threatened by Adam at one point
* DarkIsNotEvil: He's a spotted hyena but is easily one of the most sympathetic characters on the show besides Ingrid Giraffe.
* TheHyena
* {{Keet}}
* MoodSwinger: So very much.
* OcularGushers
* PuppyDogEyes: He's downright ADORABLE when he pulls it off.

'''Henry Armadillo'''
->Voiced by: TomKenny

A nine-banded armadillo with a Texas accent and a reporter in the school media.

* NerdGlasses
* TooKinkyToTorture: He seems to ''like'' everyone treating him like crap.

'''Euripedes'''

* LadyLooksLikeADude

'''Deidre Koala'''

* TheScrappy: In-universe example

'''Larry Raccoon'''

* {{Delinquent}}

'''Principal Poncherello Pegone Pixiefrog'''
->Voiced by: 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
* SmallNameBigEgo

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

* SassySecretary

'''Mr. Cyrus Hornbill'''
->Voiced by: MauriceLaMarche

* ButtMonkey
* GentleGiant

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

* [[HippieTeacher Hippie Counselor]]

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

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

* LargeHam

'''Coach Tiffany Gills'''

* LargeHam
* MobileFishbowl: teaches from a bowl wheeled around by Vice Coach Horace.

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

* CuteMute
* PerpetualFrowner
* SilentSnarker
* TheVoiceless
* WeaselMascot

'''Mrs. Eugenia Tusk'''
->Voiced by: CreeSummer

'''Nurse Jacquline Gazelle'''
->Voiced by: 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: CreeSummer

'''Ms. Bear'''
->Voiced by: TomKenny

'''Kerry Anderson'''
->Voiced by: CreeSummer

* AmbiguouslyBrown
* {{Love Interest|s}}
* PeekABangs

'''Chad'''
->Voiced by: TomKenny

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

* StalkerWithACrush

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