Jupiter Jones is the self-proclaimed First Investigator and very competent leader of the Three Investigators. He's the brains of the outfit and shows off his prowess in that area in every story. He is an orphan, lives with his aunt and uncle, and acts very mature for his age. He is always referred to in the stories as "stocky," which implies that he is a few pounds overweight but is not obese.
In several books, it is revealed that Jupiter was a child star. Just how young he was when he acted is up for debate, but since his name was "Baby Fatso," chances are that he was little more than a toddler, although he does have memories from the show. Some information on this aspect of Jupe's life is given in the introduction to "Terror Castle" and the body of "Vanishing Treasure" with more detail provided in "Rogues Reunion." However, while Jupe at least seems to have dealt with his past acting career in "Vanishing Treasure" (no hint of negative feelings when he runs into Mr. Frank), he is portrayed as being somewhat resentful of the TV show in "Rogues Reunion."
Jupiter's past acting ability benefits him frequently in mysteries because he can act older than he is, perform imitations of people when necessary (the imitation of Alfred Hitchcock in "Terror Castle" is a classic moment of humor), and act less intelligent to extract information from potential suspects (such as during his initial encounter with Three-Dots in "Fiery Eye").
Jupiter is a prolific reader and inventor and frequently invents a device that is necessary to make solving a mystery easier. Jupiter has a knack for usually seeing clues at the right angle to solve an unsolvable mystery. He also likes to play pranks (the walkie-talkie prank on Bob and Pete in the early chapters of "Whispering Mummy" and the mind-reading prank on Bob in "Fiery Eye" are some of the funniest moments in books for youths).
Because of his intellectual side, Jupe is adept at using big words and frequently uses them to his advantage, particularly to seem older, annoy Pete, and startle adults.
Jupe hates to let go of a mystery, which frequently means that he drags Bob and Pete along for the ride. But you have to admit that he always solves the case.