The show is about a kid inventor named James Isaac Neutron (voiced by Debi Derryberry ) who gets in and out of trouble by inventing far-out gadgets. Especially notable is the fact that the "brainy nerd" is the hero. His main catchphrase seems to be "Brain Blast!", which usually comes after Jimmy comes up with an idea, accompanied by a visual of the activity in his brain.
Carlton "Carl" Ulysses Wheezer, Jimmy's neurotic, llama-loving friend. According to canon, Carl was born in 1991. Carl is known as nervous, timid and suffering from hypochondria; at times, he tries to overcome these personality traits in order to appear "cooler" around Jimmy and mutual friend Sheen Estevez. When his parents expanded their cable lineup, Carl found himself focusing on a channel devoted to llamas; he has adored them ever since, going so far as to join a fan club, the Llama Love Society. Has an odd taste in women and an even stranger crush on Mrs. Neutron. Voiced by Rob Paulsen.
Sheen Juarerra Estevez, Jimmy's hyperactive friend, is twelve years old, making him two years older than Jimmy and Carl. The three are in the same school grade because Sheen was held back. Sheen's wardrobe largely consists of a bluet-shirt and jeans. The t-shirt depicts UltraLord , a popular superhero within the show. Sheen is of Mexican ethnicity, but does not have a noticeable accent. The name "Sheen Estevez" appears to be a reference to the Estevez family of actors — specifically the two members, Ramón and Carlos, who changed their names (to Martin and Charlie Sheen, respectively). Voiced by Jeffrey Garcia , who also won an Annie award for the role.
Cynthia "Cindy" Aurora Vortex, Jimmy's rival. Has shown obvious but reluctant friendship toward Jimmy. Usually calls Jimmy Nerdtron or some other such name. Voiced by Carolyn Lawrence.
Liberty "Libby" Danielle Folfax , Cindy's best friend. Somewhat intellectual, and obsessed with electronic gadgets. Also Sheen's girlfriend, by some twist of fate. Voiced by Crystal Scales .
Ms. Winifred Fowl, 5th gradeteacher at Retroville's middle school. Attracts the oddest men: was married to a werewolf at some point, and once dated Thomas Edison after Jimmy brought the famed scientist to the future. Resembles a large bird both in that she has a hooked nose and a tendency to squawk. Voiced by Andrea Martin.
Nick Dean, the "cool" kid at school, of Latino descent. Often seen on a skateboard, doing dangerous tricks. Voiced by Candi Milo.
Guest characters
Guest characters in The Adventures of Jimmy Neutron: Boy Genius are listed below:
Amber (Debi Derryberry; seldom appears)
Barbarino (Jeff Bennett; appears in episode A Beautiful Mine)
Beautiful Gorgeous, the femme fatale (Wendie Malick; appears in episode Operation: Rescue Jet Fusion)
Betty Quinlan, who also seems to have a crush on Jimmy (Kath Soucie; seldom appears)
Bolbi Stroganovsky, the resident exchange student from some Eastern European country (Phil LaMarr; appears in episode Jimmy For President et al)
Britney (Candi Milo; seldom appears)
Brobot (Paul Greenberg ; appears in the episode of the same name and later The Junkman Cometh)
Butch (Rob Paulsen; seldom appears)
Captain Betty (Jim Cummings; appears in Monster Hunt)
Chip (Rob Paulsen; appears in episode A Beautiful Mine)
Chuck Lestor (Crystal Scales ; appears in episode Hall Monster)
Clark (Mark DeCarlo; appears in episode The Retroville 9)
Emily (Debi Derryberry; appears in episode Love Potion #976/J)
General Ernest Abercrombie (Mark DeCarlo ; appears in episode Substitute Creature)
Giggles the Clown (Ben Stein; appears in episode Hypno-Birthday to You)
Gortox the Irritable (Jim Cummings; appears in episode The Junkman Cometh)
Granny Neutron (Phyllis Diller; appears in episode Granny Baby and a few other minor bits... this is her second one-shot role as a main character's elder relative, after playing Grandpa's Sister in an ep of Hey Arnold!)
Gretchen (Megan Cavanagh; appears in episode The Retroville 9)
Gus (Rob Paulsen; appears in episode Normal Boy)
Hilgo (Megan Cavanagh; appears in episode Time Is Money)
Ike (Crystal Scales; appears in episode Operation: Rescue Jet Fusion, Part 1)
Japanese Announcer (Michael Hagiwara ; appears in episode The Retroville 9)
Jensen (Mark DeCarlo; appears in episode Maximum Hugh)
