In the older novels, Jupiter is described as having messy black hair and being rather hefty. In the German extending series, The Three Investigators: Kids, his hair is brown instead.
In the later novels, he loses some weight but is still significantly heavier than Peter Crenshaw and Bob Andrews. Although he tries several diets, he always fails to keep them up for long enough, often ending up stuffing himself with food afterward.
|“||There's only one thing bigger than Jupe's IQ — and that's the national debt.||”|
—Peter Crenshaw, Long Shot
Jupiter is extraordinarily smart for his age and is considered a "genius" by many. His keen intellect is reflected in his astute deductive skills and his penchant for uncommon words, much to the dismay of his friend Peter Crenshaw, who is often stumped by his use of grandiose vocabulary. He is, however, frequently judged and even disliked for his turgid way of speaking. Although he tried to stop using those words, he ultimately found himself unable to.
Once Jupiter catches onto a mystery, he cannot give up on it until he solves it. Because of this, he often drags Peter and Bob along to cases against their will. During their investigations, he often keeps his thoughts and deductions to himself, which greatly annoys his two colleagues.
In his pursuit of justice, he is even willing to risk his life. However, if he believes that the perpetrator did not commit their crime with malicious intent, he sometimes lets them escape without any repercussions. Jupiter also abhors any form of violence.
Despite the fact that he is usually highly perceptive, he can be quite dense at times. In Mysterious Testament, he is fooled by Victor Hugenay's scheme and falls in love with Brittany, for whom he is even willing to commit theft for to pay her fake hospital bills. Jupiter also has a tendency to act hastily at times and his impertinent personality has got him in trouble more than once. In The Empty Grave, for example, he acts without thinking first because he is led to believe that his parents might still be alive.
Jupiter also has an insecure and emotional side to him. It is mentioned on several occasions that he is afraid of being less intelligent than someone else, which is why he doesn't like people like Jelena Charkova and police psychologist Mrs. Harding, who are almost as smart as him and might surpass him in the future. He is also afraid of losing his friends and family, which is shown in Message from the Underworld, where somebody proposes that he should enter the witness protection program.
According to Bob, Jupiter doesn't have much of an interest in girls. He does eventually date Qute den Zorn for a while and shows an interest in Brittany in Mysterious Testament and Rainey Fields in Funny Business but generally prefers staying single. He also often criticizes Peter and Bob for putting their relationships before their cases.
Jupiter's parents, Julius and Catherine Jones, died when he was little. Their cause of death varies from volume to volume; sometimes they died in a car crash, sometimes it's a plane crash. He was subsequently adopted by his uncle Titus Jones and his aunt Mathilda Jones and grew up on the Jones Salvage Yard in Rocky Beach.
As a child, he portrayed "Baby Fatso" in the children's TV show Wee Rogues. He is ashamed of his role in the show and often denies the fact that he ever appeared in it.
Peter and Jupiter often get into fights, usually because of Jupiter's way of speaking. They have their worst fight in Classified: UFO. Nonetheless, when Jupiter has to choose who is his best friend – Bob or Peter – in Message from the Underworld, he ultimately chooses Peter because he is thankful to Peter for saving his life multiple times.
Qute den Zorn
Qute den Zorn is a former Hollywood actress, who recently entered university. She becomes Jupiter's girlfriend in Fatal Error but they eventually break up again in The Secret Key, after she's already been absent for several volumes. His usual loyalty to Qute is challenged in Mysterious Testament, when he falls for Brittany.
Hugenay values Jupiter's deductive abilities and versatile mind. He tries to get Jupiter to work with him on multiple occasions but is only successful once. And even then, Jupiter only decides to work with him because his friends, Bob and Peter, have been kidnapped and he needs Hugenay's help to rescue them.
Although Hugenay greatly admires Jupiter, he also sees his flaws and criticizes him for coming to wrong conclusions at times.
- Deduction: His deductive abilities are far above average and he is able to think clearly even under extreme stress.
- Eidetic Memory: Jupiter can memorize even the smallest details in a short span of time. However, his eidetic memory isn't perfect.
- Acting: Due to his acting background, Jupiter is a fantastic actor. This ability often comes in handy when he has to imitate other people's voices on the phone or pretend to be somebody else to get information out of suspects.
- Gustatory Sense: Jupiter can distinguish flavors in a very precise manner and is even able to name all the different components of a meal or snack just from its taste. He can even easily tell real and artificial flavoring apart.
- Swimming: Despite the fact that Jupiter is usually terrible at sports, he is good at quickly swimming short distances. He is even better than Peter at this.
- Tinkering: Jupiter is often seen tinkering away on something and is good at repairing things. In An Ear For Danger, he can be seen fixing a walkie-talkie for somebody else. He also created a lot of the devices the Three Investigators use.
- Language skills: Jupiter is fluent in three languages: English, Spanish, and an unknown third language. He seems to suddenly lose his ability to speak Spanish in The Empty Grave, where he struggles to rent a car in Spanish.
- In the German version, his name was changed to "Justus Jupiter Jonas".
- He retains his original name in the audiobook series Die DR3I due to copyright issues.
- Jupiter was based on Sherlock Holmes, Peter Crenshaw and Bob Andrews being the Dr. Watson to his Holmes.
- He is also often compared to Holmes in the series itself. In The Mystery of the Magic Circle, both Bob and Skinny Norris call Jupiter "Sherlock Holmes".
- Jupiter also seems to be a fan of The Murders in the Rue Morgue, which features C. Auguste Dupin as the protagonist, who served as the prototype for Holmes.
- He is the white question mark.
- Jupiter has two doppelgangers: Ian Carow, who looks like him, and Brit, who sounds like him.
- In The Case of the Dancing Dinosaur, Jupiter becomes an honorary member of the wind tribe.
- Jupiter suffers from acrophobia (fear of heights), claustrophobia (fear of confined places), and nyctophobia (fear of the dark).
- In The Bite of the Beast, Jupiter buys a motorcycle but has to hide it from aunt Mathilda since she doesn't want him to drive it.