Jimmy is the main protagonist of the franchise. He is a 12-year-old boy genius with an impossible IQ of 210 and a love for science. Usually, his inventions cause more harm than benefit for the city of Retroville, but that never stops him from trying. Most of the time, he's busy saving the city from his own experiments gone wrong. He is best friends with Carl, Sheen and Nick. His middle name is named after Isaac Newton, the famous physicist.
Goddard is Jimmy's robot dog and one of his best friends. He does eleven million and four things with the exception of cleaning up after himself. Along with the ability to shoot out laser beams he has, among other things a built in ray gun, lie detector, sound system, satellite radio, computer and clock. He can also change into a flying motorcycle. Goddard is portrayed as having the habits of a regular dog with the exception of his diet of metal and that when told to play dead he self destructs in a main core implosion (only to rebuild moments later). This was a flaw that Jimmy made a feature after finding its usefulness.
Carl is Jimmy's lab assitant and one of his best friends. He is nervous, timid and suffering from hypochondria; at times, he tries to overcome these personality traits in order to appear "cooler" around the others. Carl is totally obsessed with llama and is part of a fan club, the Llama Love Society. He has an odd taste in women and an even stranger crush on Judy.
Sheen is one of Jimmy's best friends. He is very hyperactive and has a short attention span. He is still in the same grade as Jimmy and Carl because he was held back in school. Sheen is obsessed with Ultra Lord, a popular superhero and owns a large collection of Ultra Lord action figures. Sheen is named after the Estevez family of actors — specifically the two members, Ramón and Carlos, who use the screen name Sheen (Martin and Charlie Sheen, respectively). In later episodes, it is shown that Sheen cares about Libby more than Ultra Lord.
Cindy is Jimmy's rival, female counterpart and girlfriend. She is very sarcastic and constantly reminds Jimmy of his mistakes and to think his plans through, but Jimmy doesn't listen and does what he wants instead. While Jimmy and Cindy fight like cats and dogs, Jimmy still sees Cindy as a friend because she's the one person who can keep Jimmy on his feet. Throughout the course of the series, they slowly start to drop their rivalry and become better friends. She is best friends with Libby and Brittany. Cindy often gets jealous when Jimmy flirts with other girls and it usually brings out the worst in her.
Libby is Cindy's best friend and Sheen's girlfriend and female counterpart. She loves electronic gadgets and funk music. In Beach Party Mummy, she gets a makeover and keeps it. She loves Sheen, but she hates that he is always annoying her with his overbearing mentions of Ultra Lord and his occasional flirting. Libby sometimes joins Cindy in teasing Jimmy, but has full knowledge (and may have been the first to notice) that Cindy and Jimmy love each other.
Jimmy's dimwitted and bumbling father. Hugh likes pie and ducks. He loves it when his "sugar-booger", Judy, makes him waffles that resemble faces, usually ones that represent how she is feeling. He also likes every fast-food restaurant except McSpanky's, possibly because he almost became a millionaire by investing in the restaurant, but chose against it. He is the only adult who goes to the Candy Bar on a regular basis, except Sam, who runs the place. He is well known for playing with (and misusing) Jimmy's gadgets. He also invented the holiday Pule.
Jimmy's mother. Most likely where Jimmy gets his brains. Judy appears slightly more mechanically inclined than Hugh (as shown in the original movie when she repairs the family car). She is a neat freak and when angry, calls Jimmy by his full name, James Isaac Neutron. She is called "sugar booger" by Hugh. Her outfit resembles a 1950's housewife.
Ms. Fowl is Jimmy's teacher with a very vague age. Usually, when Jimmy questions the assignments, she skips ahead to some other activity. Usually when Jimmy show offs his inventions, she ends up being the victim of them (such as getting shrunk or turning into a green plant monster). She often makes various parrot noises when she talks. Ms. Fowl also hates Sheen and openly mocks him in front of the class.
He is the mean, husky and short-tempered owner of the Candy Bar, where Jimmy and his friends often hang out. His catchphrase is "Yeah". One time, it is revealed that some of his chocolate sprinkles are ants.
Nick is the most popular kid in Jimmy's class, who is also one of his best friends. Jimmy often asks Nick for advice, but he usually ends up in trouble because of it. While Nick does make wisecracks at Jimmy and tease him, Nick also serves as a big brother to him and they get along well and are pretty good friends. While Nick was part of the main cast in the movie and season one, he was reduced to a supporting character for the rest of series. He was going to rejoin the main cast in season four, but due to DNA Productions shutting down, that never happened.
Bolbi is an exchange student from Backhairistan who was first introduced in the episode, Jimmy for President. Bolbi loves slap-dancing and kabobs. He usually serves as the comic relief in season two and three, but he had a major role in his introduction episode and Win, Lose and Kaboom!
Betty is Jimmy's crush. While Jimmy likes her, Betty seems to be only flattered by his affections and she knows he belongs with Cindy. Usually when Neutron tries to impress her with his inventions, it usually causes trouble and he has to fix his mistakes.
Patrick Stewart (movie); S. Scott Bullock (pilot and series)
King Goobot is the ruler of Yolkus and Jimmy's enemy from the movie. After Jimmy stopped him from feeding the adults to their evil chicken god, he came back to Earth with his minions, pretending to have reformed so he could have everyone distrust Jimmy and then have Jimmy watch Poultra devour his friends and parents. After Goobot was defeated, he recruited several of Jimmy's other enemies to take him down. Luckily, Jimmy locked them all in prehistory and they were probably eaten by dinosaurs.
Ooblar was the self-appointed "checker of new things to be checked" on the planet Yolkus. He is the brother of King Goobot. The character no longer serves as King Goobot's assistant since he was traded for butter.
The most recurring villain of the series. Calamitous is a tiny, near-sighted, eccentric, old genius, who is one of the most brilliant evil minds on the planet, though he has a bad habit of never finishing what he starts. Calamitous is determined to get Jimmy to finish his projects for him because he can't finish them himself.
Beautiful Gorgeous is Professor Calamitous' daughter who was raised to be an evil villain and assassin from birth. She does her father's bidding since that is what she has been raised to be but has always wanted to do something else with her life. However, she can deceptive in her personality by being charming and seductive like she is towards Jet Fusion, Jimmy, Carl and Sheen when she was first introduced.
The evil and sadistic host of Intergalactic Showdown, where life forms are forced to compete against each other against their will or else their planets will blow up. Meldar is presented as a typical game show host but also a cruel, sadistic and arrogant businessman who only cares about the ratings of his show, and has no qualms with destroying entire planets if it means gaining more ratings and popularity to make his show successful.