!!James "Jimmy" Isaac Neutron
[[caption-width-right:153:"Gotta blast!"]]
->Voiced by: Debi Derryberry

The main protagonist. He is a boy with a stunning IQ and a love for science. From Hypercubes to Hypnorays, Jimmy can whip up the most amazing inventions. However, things have a way of going awry, and more often than not, Jimmy has to save the day from himself. But with friends, family, and a secret lab, there's no problem this boy genius can't handle. Right?
* {{Adorkable}}
* AesopAmnesia: At least half of the episode plots wouldn't happen if Jimmy would remember how often his inventions backfired.
* AnimeHair: Lampshaded consistently.
* BelligerentSexualTension: With Cindy.
* BerserkButton: Do NOT give Jimmy an F. [[WesternAnimation/TheFairlyOddParents Denzel Crocker]] in the [[CrossOver Jimmy Timmy Power Hour]] learns this the hard way.
* BigDamnHeroes: ALL THE TIME. Often Lampshaded, in fact.
* BigNo: In ''The Tomorrow Boys'', after learning he gets married to [[spoiler: Cindy]]. In a bit of LeaningOnTheFourthWall, the scream cuts to a commercial. When the show comes back on, Sheen comments that he had been screaming for four minutes straight.
* BlackSheep: In one episode, most of the Neutron extended family views him as such, correctly pointing out that, while Jimmy may have saved the day quite often, more often than not, it was one of his inventions that [[NiceJobBreakingItHero caused the problem in the first place]].
* BrilliantButLazy: Why do something as simple as neatly put away your piles of pants when you can implant chips in them that do it for you? Carl and Sheen call him out on this, though.
* ButtMonkey
* CassandraTruth: In the episode, "The Eggpire Strikes Back", no one believes Jimmy when he says that the [[spoiler: Yolkians, the villains from the pilot film, are still just as evil as ever,]] until Poultra is bearing down on him.
* CatchPhrase: Originally "Gotta blast!" "Think, think, THINK!" and "Brain blast!". The show eased off on these as it went on, eventually getting rid of them entirely. (One exception is "And now, my latest invention, the (insert plot-related-device here!)", which stayed for most of the series' run.)
* ChildProdigy
* ComicTrio: He's the straight man.
* CrazyPrepared: His watch, whose powers depend mainly on the plot. Can include lasers, freeze rays, magno-rays, GPS, tracking devices…you name it, there's probably a gadget for it tucked away in there.
* CrossDressingVoice
* DeadpanSnarker:
--> '''Jimmy''': *science babble about a spider*
--> '''Sheen''': Fascinating! Can you make it do the hokey pokey?
--> '''Jimmy''': Sheen, go home.
* FatalFlaw: Jimmy's greatest flaws are a combo of {{Pride}} and his lack of common sense; in fact, many of the conflicts in the show are born out of this.
* FutureLoser: In the 'BadFuture' of ''The Tomorrow Boys'', Future!Jimmy is a slack-jawed loser, wanted by the evil dictator [[spoiler: Libby]], with a five-o'clock shadow and [[spoiler: Cindy]] as a wife. Current!Jimmy is [[FutureMeScaresMe less than impressed]].
* GadgeteerGenius
* [[SheCleansUpNicely He Cleans Up Nicely]]: When Jimmy shows up in a tux for Jet Fusion's wedding to Beautiful Gorgeous in ''My Big Fat Spy Wedding'', Cindy promptly gets little hearts in her eyes.
--> '''Cindy''': "So Neutron's the best man? Yeah, more like the best NERD! I bet he'll look like a total…" (''Jimmy shows up outside the church in a tux. Cue fade to pink background, eye-hearts and hair patting'') "…HUNKMUFFIN."
** Another example is during ''Love Potion #976-J'', when Jimmy cleans up for his date with Cindy. Said almost word-for-word by Cindy.
* HellYesMoment: Often. Especially when he's given up all hope... and then his friends forgive him for being arrogant/apologize for being stupid/come back to save him.
* TheHero
* InsufferableGenius: At times, but often more pronounced when Cindy's around.
* IntelligenceEqualsIsolation
* KidHero
* LittleProfessorDialogue: Justified partially in that he does actually have an IQ of ''210''.
* NiceJobBreakingItHero: 99 out of 100 times, his inventions end up causing problems for everyone in Retroville, leading him to fix what he broke. As revealed in one episode, the majority of the Neutron extended family views him as the BlackSheep as a result.
* PetsHomageName: Jimmy designed and built his robot dog and named him Goddard in honor of rocket science physicist and inventor Robert Goddard. Especially apt when, in his first animated feature film, Jimmy is able to ride upon Goddard like a space bike.
* PuppyLove: With Cindy.
* RidiculouslySuccessfulFutureSelf: In the 'good future' of ''The Tomorrow Boys'', Jimmy is the first person to ever win the Nobel Prize in bulk.
* RivalTurnedEvil: In ''The N-Men''...
* ScienceHero
* SesquipedalianLoquaciousness: A frequent user and abuser.
* SlapSlapKiss: With Cindy. All the time.
* TheSmartGuy: Duh.
* TookALevelInDumbass: When he accidentally drained his brain too much in ''Normal Boy''.
* TVGenius

!!Carlton "Carl" Ulysses Wheezer
[[caption-width-right:210:"You look like a couple of intelligent young men!" [softreturn] "Nuh uh, it's just the glasses."]]
->Voiced by: RobPaulsen

Jimmy's shy, asthmatic best friend who has an unhealthy obsession with llamas and Jimmy's mom.
* {{Adorkable}}
* TheChewToy
* ComicTrio
* CowardlySidekick
** CowardlyLion
* BigEater: You'd think having so many allergies would limit this, but no. Carl can chow down with the best of 'em and has a stomach of steel. Occasionally overlaps with ExtremeOmnivore for laughs:
-->'''Carl:''' I don't eat newspapers! '''(Beat)''' Well, I'm trying to quit.
* FatBestFriend
* {{Fartillery}}: Gets this in "The N-Men" (see {{Gasshole}} below).
* {{Gasshole}}: He has burped more than anyone else on the show (except for Jimmy's dad). In "The N-Men", he gets hit by some cosmic radiation that turns him into "Belch Boy", allowing him to unleash [[MakeMeWannaShout sonic blast]] burps by drinking seltzer or shaking himself really hard.
* GreenEyedMonster: Implied quite heavily in "I Dream of Jimmy" and "Carl Wheezer, Boy Genius" that [[IJustWantToBeYou he'd really like to have Jimmy's life.]]
* TheHeart
* LikesOlderWomen: Carl has a crush on Jimmy's mom since ''Love Potion #976-J''.
** He's also attracted to the class's teacher, Miss Fowl, who's in her fifties. At ''least.''
* LooseLips: Sheen helps him a lot with this one. They're often the ones to reveal the secret plan to the enemy, generally via SuspiciouslySpecificDenial.
* NerdGlasses
* NerdyInhaler
* PrecociousCrush: On Jimmy's mom.
* ShrinkingViolet
* SicklyNeuroticGeek: What else can you expect with a surname like 'Wheezer'?
* StalkerWithACrush
* TookALevelInJerkass: Carl takes it in "Carl Wheezer, Boy Genius", which starts out as a standard "I lied to my pen pal" farce, then quickly escalates as we learn Carl lied about having ''Jimmy's'' life. It goes downhill as Carl not only has to masquerade as a boy genius, but he forces Jimmy to be his "lowly assistant", berating and constantly screaming at him for all ''his'' mistakes. Even by the end, when Jimmy has saved his life at least three times, Carl is ''still'' screaming at him... and projecting his own insecurities onto Jimmy to boot.

!!Sheen Juarrera Estevez
[[caption-width-right:300:"Back, or I will slay you with my medulla oblongata!"]]
->Voiced by: Jeffrey Garcia

Jimmy's hyperactive other best friend who is obsessed with the comic book superhero Ultra Lord.
* AccentAdaptation. In the Latin American dub, he speaks with a Japanese accent, possibly because the translators thought that he's Japanese-American, instead of Hispanic, and his name should be pronounced "Shin".
* {{Adorkable}}
* BetaCouple: With his sweet princess, Libby.
* TheBigGuy
* {{Cloudcuckoolander}}: In a minor but interesting play off of the idiot sidekick shtick. Sheen isn't exactly stupid (at least before Flanderization set in); he's just very, very scatterbrained.
* ComicTrio
* TheDitz: His IQ is 32.
* DrunkWithPower: When his intelligence is increased in ''Sheen's Brain''.
* EarlyInstallmentWeirdness: He mentions a sister in "I Dream of Jimmy" but WordOfGod has said since to disregard it. Canonically, Sheen is an only child who lives with his father and grandmother (at least until ''WesternAnimation/PlanetSheen'').
* ExtravertedNerd
* GoofyPrintUnderwear: Limited Edition Ultra Lord Underwear! Only available in Japan!
* HollywoodToneDeaf: His singing is so bad, it actually puts the music-hating Twonkies to sleep.
* {{Keet}}
* LoonyFan: He's obsessed with cartoon superhero Ultra Lord.
* LooseLips: See above.
* MissingMom
* PluckyComicRelief
* PuppyLove: With Libby.
* TheSlacker
* SpinOff: Gets his own one of these, ''WesternAnimation/PlanetSheen''.
* TheWatson: Has his moments.

!!Cynthia "Cindy" Aurora Vortex
[[caption-width-right:210:"Listen, Neutron, I'm a girl with looks and brains and big plans for my future and they ''don't'' include living in a world ruled by pants!"]]
->Voiced by: Carolyn Lawrence

Jimmy's rival who has a crush on him. Not that she'll admit it.
* AcademicAlphaBitch
* ActionGirl
* BareYourMidriff: As Special Girl.
* BelligerentSexualTension: With Jimmy.
* BlondesAreEvil: While not evil per se, she can be a real {{Jerkass}}.
* CannotSpitItOut: More because of pure stubbornness than anything, really.
* ClingyJealousGirl: She doesn't like seeing Jimmy showing interest in other girls.
* DeadpanSnarker
* DefrostingIceQueen
* GeniusBruiser
* GirlishPigtails: In the movie.
* GoofyPrintUnderwear: Polka dot panties.
* GreenEyedMonster: At least part of her problem with Jimmy stems from this. In The Movie, she says that before Jimmy came along, ''she'' was the smartest kid in school. It's a given that she didn't appreciate being knocked down to number two.
* HairOfGoldHeartOfGold: She really does care about Jimmy deep down, though she's more in the disguise of JerkWithAHeartOfGold.
* HintDropping: Unfortunately, Jimmy seems to permanently be holding the IdiotBall when it comes to interpersonal relations.
* HypnotizeThePrincess: Gets hit with this in the Halloween special, courtesy of Vampire!Carl.
* IKnowKarate: She actually says this in "King of Mars."
* {{Jerkass}}
** JerkWithAHeartOfGold
** JerkassFacade: She's mostly a real jerk to Jimmy because she has a crush on him.
* KungFuKid: Cindy practices karate. She goes to competitions, is sometimes seen in her gi, and although we're never told what level she's at, based on some of her flips, she's probably a black belt.
* TheLancer
* LovingBully: She picks on Jimmy but she only does this to mask her love for him.
* PuppyLove: With Jimmy.
* TheRival: To Jimmy.
* SlapSlapKiss: With Jimmy. Are you starting to get an idea of their relationship yet?
* TomboyishPonytail: In the TV series.
* {{Tsundere}}

!!Liberty "Libby" Danielle Folfax

[[caption-width-right:248: "Hey Corky, you're going to need some hot funk for that show and I'm the girl who can bring it!"]]
->Voiced by: Crystal Scales

Cindy's best friend. The love of Libby's life? Music, the funkier the better. And she doesn't just listen, she also plays, DJ's, and has pipes that would make most opera singers sick.
* BetaBitch
* BetaCouple: With Sheen.
* BlackBestFriend
* TheChick
* {{Fartillery}}: Gets this when she ate the Belch Boy packet in "The League of Villains" and proves to be ''better'' at it than Carl, given that she doesn't need seltzer or to be shaken to burp, and she can eventually control her burping to do it at will.
* {{Gasshole}}: Becomes one after eating the Belch Boy packet in "The League of Villains", even to the point of making a ''dance'' out of her burps.
* GiveGeeksAChance
* ImportantHaircut: Libby gets one in the ''Beach Party Mummy'' episode to fool a bunch of mummies that were chasing the gang. She keeps the new look for the remainder of the series.
* IncrediblyLongNote: Turns this into a ChekhovsGun in "The Great Egg Heist."
* OnlySaneWoman: Grows into this more and more as the series went on. By the end she's the most levelheaded character in the gang, much to her occasional frustration.
* PromotedToOpeningCredits
* PuppyLove: With Sheen.
* SassyBlackWoman
* SatelliteCharacter: Was originally one for Cindy, and several of her earliest appearances had her personality as more or less an echo of Cindy's, but as the characters got more development, she became more distinct.
* ShipperOnDeck: She's FOREVER pleading with Cindy to just tell Jimmy how she feels and get it over with!
* TokenMinority
* {{Tomboy}}: She adjusts really quickly to becoming "Burp Girl" in "The League of Villains", even proving better at using the sonic-blast burps than Carl did in "The N-Men".
* WomenAreWiser
* YouthfulFreckles: Libby gained some in season 2 after her redesign.

->Voiced by: Creator/FrankWelker

Jimmy's robotic dog.
* DoAnythingRobot: He does eleven billion and four things! Except clean up after himself.
* NonHumanSidekick
* RobotBuddy
* RobotDog
* TheSpeechless
* TeamPet

!!Hubert "Hugh" Beaumont Neutron
[[caption-width-right:195:"Hi, Puffmomma. Remember how I promised I'd never be brought home by an angry mob again?"]]
->Voiced by: Mark [=DeCarlo=]

Jimmy's father. Averts Father Knows Best. Nevertheless, he's a chipper, upbeat family man who always has time to dispense words of wisdom for Jimmy, even if most of the time those words are better left ignored. Has an obsession with ducks.
* AdultsAreUseless
* BumblingDad
* {{Cloudcuckoolander}}: Quite possibly the largest one in the series.
* TheDitz
* {{Gasshole}}: Even without drinking the Burping Soda, Hugh has burped more than anyone else in the series, including Carl.
* IdiotBall: Holds it consistently, like it's his firstborn child.
* JumpOffABridgeRebuttal: Twisted.
--> '''Jimmy''': "Aw, but Dad! All my friends are going!"
--> '''Hugh:''' "Now, Son, [[IceCreamKoan if all your friends were named Cliff, would you jump off them?]]"
* ObliquelyObfuscatedOccupation: Averted. He's a car salesman.
* TrademarkFavoriteFood: Pie.
* UglyGuyHotWife: With Judy.

!!Judith "Judy" Neutron
[[caption-width-right:300:"To think, I stepped out for two measly days and you replace me with some psycho robot with a hideous 1950's hairdo?"]]
->Voiced by: Megan Cavanagh

Jimmy's mother. She's happy to cook and clean and run the household but does want some appreciation for all she does. After all without her, Jimmy and Hugh's lives fall to pieces.
* ActionMom
* AdultsAreUseless: Subverted. She's incredibly athletic, coordinated, intelligent, and she keeps a plasma rifle in her utility closet (for those tough ground-in stains!)
* BeautyMark
* DitzyGenius: Not ditz per se (she's certainly the intelligent one in her marriage with Hugh and more rational than he is) but she has her moments of ditziness. There are also numerous moments where she demonstrates it's definitely from her side of the gene pool that Jimmy gained his intellect.
* FullNameUltimatum: "James Issac Neutron!"
* {{Gasshole}}: Becomes this after chugging some of Jimmy's Burping Soda.
* HypnotizeThePrincess: Ever noticed she gets hypnotized quite a bit on this show?
* MamaBear: "'''Get away from my family, you ugly bucket of bolts!'''"
* MundaneUtility: Use a demolecular cannon beam to clean difficult stains.
* OnlySaneWoman
* StacysMom
* UglyGuyHotWife: With Hugh.
* WomenAreWiser

!!Nick Dean
[[caption-width-right:215:"You know, these are as cool as... me!"]]
->Voiced by: CandiMilo

The "cool guy" that everybody loves at school, especially the girls.
* AllGirlsWantBadBoys
* BadButt: Before his later characterization took hold.
* CrossDressingVoice
* DemotedToExtra: Earlier episodes were building him up to be an important character. Then he vanished.
** He even got his own toy in the Wendy's Kid's Meals when the original film came out.
* {{Flanderization}}: In the movie, Jimmy takes backseat as Nick leads the charge against the Yolkians, and the class BadButt even manages to take out a couple of Yolkian guards by hijacking one of their spears... only to lose momentum upon seeing the giant alien-chicken, at which point he becomes completely useless and Jimmy has to lead the charge himself. Nick's later Paper Tiger characterization suggests he would balk at even taking on the guards, if he had to do it again.
* PaperTiger: He's the ultimate cool dude, the girls all dig him, and he's supposed to be tough. In the movie, his part of the plan is to take care of the BigBad while the others rescue their parents. One look at his competition, though, and he goes screaming like a little girl.
** And it only goes downhill from there…
* ScreamsLikeALittleGirl

!!Bolbi Stroganovsky
[[caption-width-right:210:Vote for Bolbi!]]
->Voiced by: Creator/PhilLaMarr
* {{Cloudcuckoolander}}: He makes ''Sheen'' look sane by comparison.
* DanceSensation: The "Slap Dance" from his homeland. [[EarWorm "Slap slap slap! Clap clap clap! Slap slap slap! Clap clap clap!"]]
* FunnyForeigner: Kind of like [[Series/ThatSeventiesShow Fez]] or [[WesternAnimation/EdEddNEddy Rolf]]. The difference is that Bolbi's country is actually named ([[PunnyName Backhairistan]]).
* HiddenDepths: He's a very good actor.
* StrongFamilyResemblance: His sister is just a slightly shorter version of him in blonde braids.
* TheSixthRanger: Starts taking a role similar to this in later episodes.
* TrademarkFavoriteFood: Kebabs. They appear to be standard cuisine in his country.

!!Betty Quinlan
[[caption-width-right:248:"Betty is a woman."]]
->Voiced by: Creator/KathSoucie

The girl of Jimmy's dreams, and sometimes the motivation for hi jinks in his effort to impress. Cindy doesn't care for her... for ''some'' reason.
* BareYourMidriff: When she played Lady Macbeth.
* BettyAndVeronica: Quite literally the sweet natured Betty to Cindy's Veronica.
* BeautyMark
* ShallowLoveInterest: For the most part. She doesn't get any true focus outside of Jimmy's love glazed POV. Finally subverted a bit when we see, that despite her pleasant demeanor, she ''does'' have her limits. And no, she's ''not'' oblivious to Jimmy's crush on her (she'd have to be deaf, dumb, and blind, actually), or Cindy's crush on Jimmy for that matter. [[spoiler: She lets Cindy have him.]]

!!The Twonkies

* BerserkButton: '''''MUSIC'''''. Seems to have stopped after their first appearance, though, as one can be seen during a big musical number at one point.
* FishEyes
* KillerRabbit
** VillainDecay: After their first appearance, Twonkies are treated as if they were normal rabbits.
* MonsterOfTheWeek --> RecurringExtra

Owner of the local hangout joint, The Candy Bar.
* BullyingADragon: Does so in the superhero episode the N-Men. Jimmy is a [[IncredibleHulk rampaging orange monster]] but upon seeing his parents, he starts calming down. Until Sam [[TooDumbToLive decides to throw fruit]] in Hulk/Jimmy's face. Needless to say, the mutated Jimmy doesn't take too kindly to it.
* {{Jerkass}}: N-Men episode again. Granted something should have been done to try to help the kids control their new found powers. However, calling the military up to ''imprison'' them and then telling the kid's parents they were just ''freaks''?
* VerbalTic: Has a habit of ending his sentences with, "Yeah!".

!!King Goobot
->Voiced by: Creator/PatrickStewart (movie), S. Scott Bullock (TV series)

King of the Yolkians, and Jimmy Neutron's archenemy.
* ArchEnemy: To Jimmy.
* BigBad: Of the movie.

!!Professor Calamitous
->Voiced by: TimCurry

Jimmy's arch-nemesis and the show's most recurring antagonist.
* AbsentMindedProfessor: Calamitous when introduced was never able to finish anything, even his own sentences. From his second appearance onwards, he can now talk in complete sentences thanks to self-help classes.
* BigBad: Expect him to fall into this role a lot, especially during crossovers.
* EvilLaugh
* InTheirOwnImage: After being fused with [[WesternAnimation/TheFairlyOddParents von Strangle]], he intended to recreate all reality as he saw fit.
* MadScientist
* MiniMecha: A humanoid red model was his best work in his debut. The only thing he hadn't finished was putting in a bathroom.
* WouldHurtAChild


!!Beautiful Gorgeous
->Voiced by: Wendie Malick

Professor Calamitous's daughter. Jimmy, Carl, and Sheen are easily seduced by her looks.
* BeautyMark
* BeleagueredAssistant
* DeadpanSnarker
* DudeMagnet: How every guy in the show reacts towards her.
* ImpossibleHourglassFigure
* InterspeciesRomance: With Junkman due to Jimmy's LovePotion.
* MadScientistsBeautifulDaughter
* MeaningfulName
* MsFanservice
* RunningGag: Every time she mentions she wanted to have a certain career other then villainy, she always gets interrupted or cut off. [[spoiler: It was to make [[WhatTheHeckIsAnAglet aglets]] for a living.]]
* ThenLetMeBeEvil: Apparently she never wanted to be evil; just felt obligated to because her father was doing it.
* TheUglyGuysHotDaughter
* UglyGuyHotWife: With Junkman.
* WomanInBlack

->Voiced by: Creator/TimAllen

The host of the hit game show "Intergalactic Showdown" and [[BigBad the main villain]] of the TV movie "Win Lose and Kaboom".
* BigBad: Of "Win Lose And Kaboom".
* DoingInTheWizard: His seemingly supernatural hold over reality, time and space is actually caused by a pair of matrix generators hidden in his cuff links.
* FauxAffablyEvil: Rarely drops his cheery, hammy attitude even while threatening to ''blow up people's planets''. At the end, he finally stops and decides to [[LetsGetDangerous get serious]].
* {{Hypocrite}}: Insists during the race that there's no cheating allowed, but permits the Brains to put Jimmy and the gang into a LotusEaterMachine. Later he panics at the idea of his quadrant being blown up despite the fact that his entire show is about blowing up completely innocent planets simply because their species [[DisproportionateRetribution lost a game]].
* KarmaHoudini: The worst he gets is being forced to hand over his show to the Brain aliens and advertise an exercise machine that implements ElectricTorture. Probably justified as he's implied to be indestructible.
* LargeHam: He's a GameShow host. It comes with the territory.
* OmnicidalManiac: "Winner gets a brand new car! Loser gets their home planet ''destroyed!''"
* TheOmnipotent[=/=]RealityWarper: Thanks to the matrix generators in his cuff links.
* SmugSnake
* TimeAbyss: Implied, as April states that his show has existed for eons.
* VillainousBreakdown: Loses it when he discovers that his show is about to be destroyed.