James Isaac "Jimmy" Neutron (Voiced by Debi Derryberry) is the main protagonist of the televsion series and movie. He is the world's smartest 11-year-old. He is named after scientists Sir Isaac Newton, and James Chadwick who was nicknamed "Jimmy Neutron", because of his discovery of the neutron. Jimmy is a genius having an I.Q. of 210. In a number of episodes, his classmates are annoyed and angered because Jimmy's brilliance renders trying to win projects and competitions things pointless. (In fact Jimmy has been banned for life from the science fair to give normal kids an actual chance). This brain, a source of constant comment, allows him to invent devices that accomplish even the impossible. That might seem like a gift to most people, but to Jimmy, it often causes him trouble. He designs his wild inventions to help him deal with bothersome chores, however the inventions tend to have a flaw (usually caused by an obvious oversight) which result in a catastrophe which only he can solve. He's rarely ever thanked however as people usually know he's the cause of the problem in the first place.
In the moment of crisis, he will urge his brain by saying, "Think, think, think!", where upon viewer enters Jimmy's ear and goes into his brain to observe its inner workings as well, as to relive a scene from the early part of the episode. Jimmy will then shout, "Brain Blast!", and arrive at the solution for whatever danger he has landed himself, his friends, or his town in. In the movie, he does not say "Think, think, think!" or "Brain Blast!" out loud, but says them in his mind. In the third season, he seems to stop brain blasting and comes up with ideas right away, with the exception being "Fundemonium".
Jimmy has a sweet personality, although sometimes he can be a bit arrogant. Jimmy is shown to be quite irritable around most people, the exception being his friends. He can be friendly at times, that he needs (or wants to be), but can come off as a smart alec or nerd occasionally. He enjoys playing the hero, often taking a staunch stand against anyone he believes to be evil. He won't back down from his beliefs as he strongly believes in himself. Jimmy is quite charming when he least tries to be, such as at the party, that he throws at his parents' absence in an early episode. He cares for his friends, family, and Retroville.
- Jimmy will meet Jeffrey and his team in Jeffrey & Friends meet Jimmy Neutron: Boy Genius.
- Jimmy will join the Cyberchase Fighters in Michael Buzz and Delete meet Jimmy Neutron Boy Genius.