The series reimagines the title character Martin Mystery and his stepsister Diana Lombard (after Martin's mother's death, Martin's father married Diana's mom) as 16-year-olds attending Torrington Academy, a high school in Sherbrooke, Quebec. They work for the covert organization "The Center", which secretly protects the people of Earth from extraterrestrial and supernatural threats. Like all secret organizations, they must keep their work secret, thus anyone who comes too close to their missions, i.e. seeing a monster, will have their memories of the event erased. Not everyone who works for the organization is human. The short and green-skinned alien, Billy, is one of Martin's best friends. Java works as a lunch server of Diana and Martin's school and is a caveman from 200,000 years ago; he also aids Martin and Diana on their missions. As far as "The Center" is concerned, Martin's vast knowledge of the supernatural and his remarkable intuition compensate for his poor personal hygiene and huge ego. Diana resents these qualities at times, as Martin often does not seem to understand the seriousness of his missions, but generally loves him like any real brother. Diana and Martin are lovers, then married, in the original comics version, and not step-siblings.
Martin is just like any other 16-year-old, and has a wild passion for the paranormal, but after unwillingly solving an "amazing out-of-this-world mystery," he finds himself working as a paranormal investigator for "The Center." Whether at school or on a mission, he always remains the same outrageous immature teen, constantly bursting with energy. He seldom thinks before he leaps into danger, often to help someone in peril. He is an underachiever at school, once getting 106 out of 902 in his report card results. Martin has a peculiar love for everything slimy, sloppy and gooey - things that make Diana gag. Martin loves to play pranks and creep out his stepsister, who often retaliates with physical abuse. He is also seen in Totally Spies saying that Sam is just like his sister which makes her mad. As the leader of the trio, Martin is also the only one who wears a U-Watch. Martin's favorite hobby is teasing Diana. A running gag in the series is Martin falling in love with every good-looking girl he meets, who usually reject him by using verbal or (in Jenni's case) physical abuse. Voiced by Sam Vincent.
Diana is a 16-year-old student with nothing in common with her step-brother. As an agent at "The Center," she strives to bring more seriousness into Martin's life, and is Martin's female counterpart during investigations. Diana is at times quite jumpy and squeamish on missions, but shows great courage when called on. Not sharing Martin's love of the paranormal, she often tries to explain something with logic before it becomes apparently paranormal. Diana has the hardest time keeping her cool with her irresponsible step-brother, and will generally slap, kick, beat or scream at him quite often. Regardless of Martin's quirky behavior, she loves him like her own real brother. She is usually seen with her two signature hair clips and her small purple shoulder bag. She also seems to be the bait on the majority of the missions and the "damsel in distress". Her best friend is Jenni Anderson. Diana usually loses bets to Martin. She is probably the second or first victim in the episodes including being cocooned by an acid spewing insect queen, turned into a frog by a warlock Martin unleashed, and being possessed by a vengeful spirit at a mountain lodge. When Martin is transformed by the monster, Diana uses her own skills to save him. In few episodes Diana wears Martin's U-Watch and uses it when he loses it while being transformed. Voiced by Kelly Sheridan.
Java is the 200,000 year-old caveman, a friend and aide-de-camp to Martin and Diana who works at Torrington as a cook and a janitor. He assists them in their investigations, with his brute strength being useful when battling monstrous foes or breaking through barriers, and also serves as the team's tracker. A lover of lizards and spiders, Java nevertheless has fears of cats, heights, and technology (his fear of technology being a major joke in the show). Also the member in the trio who speaks the least, his English is spoken on a fairly simplistic level (i.e. "What that?", "Diana bald", and "Java no like this place"). Also, Java has a knack of thinking outside the box which greatly helped in the team's adventures. Java is, however, a significant source of comedy in the show through his reactions to various scenarios, and various situations related to his poor hygiene as a caveman (i.e. his offensive foot odor, stinky armpits and filthy breath). Diana once had to give Java foot massage after losing a bet to Martin. Java is sometimes the victim of being transformed into a monster, such as inhaling the Chaos Beast in its gas form, broken into pieces by the Djini (because he wanted to fit in better with the modern world) and brainwashed by a pirate controlled Martin. Voiced by Dale Wilson.
As the multi-tasking director of "The Center", she oversees the investigation of paranormal activities throughout the world. Martin's amazing capabilities make him one of M.O.M.'s favorite agents - i.e. if "favorite" is meant by the agent that she least likes to have appear in her office. Her real name is unknown, though she has used the alias Ms. Olivia Mandell in episode "Web Of The Spider Creature," which is derived from her initials of M.O.M. Before working at the center, she used to drive an ice cream truck. Serious uncompromising and an expert at hand to hand combat, she has to deal with Martin blowing up just about every piece of gadgetry she is working on whenever he appears for a debriefing in her office which was a running gag until season 3. Despite her typically dry sense of humor, she is actually quite- as Billy says- "a real party animal"- which is seen in some pictures Billy shows Martin that M.O.M. parties like the drunk chick at the party. When she was an agent, her first ever creature captured was Gastromo, a parasitic, iron eating monster who infects and controls those he touches. In the crossover episode of Totally Spies! titled "Totally Mystery Much?", M.O.M. is shown to have a history with the spies' boss Jerry that appears to have romantic overturns much to the spies and Martin's displeasure as she and Jerry spy-flirt in front of Alex and Martin. The initials M.O.M. are said to stand for "Mystery Organization Manager" as explained by Martin in "Totally Mystery Much?". M.O.M. is voiced by Teryl Rothery.
A tiny green alien who works for "The Center" as M.O.M.'s personal secretary, often seen flying on his mini hovercraft. He is a powerful idolizer of Martin and in one episode, it was revealed that in his room, he even had a picture of Martin, making him the "crazed fan of the show" type character. His role is generally a supportive one; he pops up anytime during a mission to give Martin, Diana and Java valuable information, usually startling them in the process and he does analysis on slime or fabric samples sent by Martin- but when he is at the mission. He won't help Martin and the gang on a mission directly unless forced. In the second season, Billy was revealed to have once been a great alien warrior, known as Ganthar, on another planet, but gave up violence for good- when Martin asked why a planet on Billy's list of hologram worlds was blue, it was said by M.O.M. that that was the world which changed Ganthar into Billy, which is the reason he doesn't go on missions with the other three agents. He arrived on Earth in the Roswell UFO Incident in 1947. Billy recently adopted a human disguise—the "B.S. 1000"— to be with Martin and Diana at their school. A few episodes in season 3 hinted that he liked Diana, such as in one of the Christmas episodes he invited her under the mistletoe and actually went on a friendly date with her. It is revealed in season one episode "Revenge of the Doppelganger" that Billy was once a super model on his home planet. This might be true, since he was a powerful, fearsome and might have been popular with his people. Voiced by Samuel Vincent.
Jenni is Martin's constant—and rather unrequited—love interest. Her view of Martin is generally not very flattering: this includes "immature", "freak", or "loser". Jenni is also one of the prettiest and most popular girls attending Torrington, her appearance being that of blue eyes, flared auburn-colored hair, black blouse and skirt, and knee-high boots. She is also Diana's best friend. Though Jenni usually sees Martin as a "brat", there have been times where she is actually interested pursuing a relationship in him- when Martin acts like a gentleman. Unfortunately for Martin, though, it never lasts, because her mind is erased of what ever happened, or Martin does something stupid, or Jenni moves for a semester, or some other similar incident occurs. Voiced by Tabitha St. Germain
Marvin is Martin's rival, and evidently one who does extreme things (such as "snowboarding down K-2"). He does virtually everything that Martin himself can do (better, of course), and even dresses in a style similar to him. Marvin's similarity-yet-dissimilarity to Martin may be a nod to Bizarro's resemblance to yet difference from nemesis Superman. His father is a pilot, allowing Marvin to travel around the world on a frequent basis. Marvin first appeared in "Attack of the Mothman" where he shows off his skateboarding skills and his very own comic book. His talents make Martin jealous as Diana, Java, Jenni, and even Billy fawn over him. He reappears in "The Amazon Vapor" where he discovers Martin and Diana's secret and helps them on their mission. In "The Third Eye", he works for the Center and has acquired his own U-Watch much to Martin's dismay and Diana's delight. Unfortunately for him, after the mission, he ends up being deported to the Center's South Pole facility and blames his unfortunate circumstances on Martin. He continues to hold a grudge against Martin (and Martin still stands strong at his hatred of Marvin) throughout "Body-Swappers" until Martin gives him the majority of the credit for getting rid of the parasite and M.O.M. decided to relocate him to Hawaii- much to Martin's dismay and much to Marvin's elation. In a subsequent episode, "Rage of the Leprechaun", Marvin contacts Martin and Diana for help after he was being stalked by a leprechaun that wanted to get back the magical four leaf clover that made anyone who held it incredible lucky that Marvin had taken from him. Which makes Marvin unlucky because he took it rather than received it, which makes a person lucky. After the incident, Marvin is chewed out by M.O.M. for keeping the clover instead of getting rid of it. What happens to him after that is unknown, even at the end of the series. Voiced by Matt Hill.
Gerard Mystery is Martin's father and Diana's step-father. He married Diana's mother when Martin and Diana were very young. Unlike his immature son, Gerard is a very rational, logic-minded person like Diana and considers anything related to the paranormal to be "balderdash". As a scientist, he spends much of his time in the forest doing field work. In the third season, he finds out about "The Center" and that Diana and Martin work for it. Voiced by Michael Donovan.