Jimmy is a 10-year-old boy genius. He has fair skin, a disproportionately large forehead, blue eyes, and brown hair in a swirly cowlick. He is often teased because he is shorter than most boys of his age, though sometimes he appears to be the same height as his friends. Jimmy's wardrobe consists almost exclusively of red T-shirts that sport his trademark symbol of a yellow atom.

Jimmy is a very intelligent kid, having an impossible I.Q. of 210. In many episodes, his classmates are angered because he is so smart and keeps outdoing them in anything math and science-related. This brain, a source of constant comment, allows him to invent devices that accomplish even the impossible. This usually leads Jimmy causing accidental trouble to Retroville. He designs his wild inventions to help him deal with bothersome chores, but they often end up causing a bigger problem. They do put him in exciting adventures though, and make life more interesting.