A teenage guy who has a wild passion for monsters and the paranormal, but after unwillingly solving an "amazing out-of-this-world mystery," later revealed to have been a mutant humanoid chameleon named Venoso, he finds himself working as a paranormal investigator for "The Center."
- Agent Mulder: His mind always defaults to supernatural causes whenever the trio encounter something out of the ordinary. Something supernatural usually is involved, though the actual theories he throws out are usually fairly off-base.
- Age Lift: In the source material, he's a college professor. This show makes him a teenager.
- Alliterative Name
- Brother-Sister Team: With Diana.
- Book Dumb: More and more as the series goes on. One later episode is even dedicated to how he's brilliant and learns much in the field even though he does nothing but fail in class.
- Casanova Wannabe: The guy constantly hits on whatever girl strikes his fancy. He's always shut-down, sometimes quite violently.
- Cloud Cuckoolander: To Diana's undying chagrin - though he always gets focused when the mission needs him to be.
- Crouching Moron, Hidden Badass
- Genius Ditz: Par excellence. His overall behavior in the series in his school life points towards an incredibly poor record in-school, but he can read Ancient Egyptian among other talents he displays in the field. It is apparent that he just has no retention of anything outside his field of interest.
- Jerk with a Heart of Gold: Despite acting like your typical hormone-addled, self-centered teenager most of the time, at his core he's repeatedly demonstrated to be a fundamentally decent and heroic person. Best illustrated when a Jackass Genie uses her powers to force the cast to voice their deepest desires so she can twist those wishes in order to make their lives a living hell. Java wishes to fit in better (and gets turned into a living jigsaw puzzle). Diana wishes to be the smartest person in the world (and suffers a severe case of My Skull Runneth Over). Martin? Even in his brainwashed state, he takes two glances at his friends' conditions and wishes he knew how to save them.
- Keet: The guy is very enthusiastic about his line of work.
- Mr. Fanservice: Despite everyone else in the story claiming otherwise.
- Sibling Rivalry: With Diana, of course. On his part, he tends to prank and tease her quite a bit.
- The Trickster: It's his brains rather than his brawn that usually allow him to defeat the Monster of the Week.
- Action Girl: It depends on the episode, but she's usually athletic enough to keep up with Martin.
- Agent Scully: Is the first to attempt to find rational explanations whenever the trio stumble across something out of the ordinary, usually to counteract Martin's wild conspiracy theories.
- Cute Monster Girl: In the season three finale, Diana gets mutated into a very cute lizard-girl.
- Hair-Trigger Temper: Pretty much any time Martin does anything remotely goofy or odd, expect her to express obvious irritation if not full-blown anger. In the very first episode, when Martin spills a bit of chocolate on her shirt, she proceeds to start beating him up in the middle of the cafeteria.
- Jerkass: Less of a jerk than Martin, mostly because her jerkassery is generally centered around Martin himself and is sometimes but definitely not always, or even usually, provoked.
- Jerk with a Heart of Gold: On her best days, she can be a decent person when you don't mess with her
- Not So Above It All: While she's typically the more mature and responsible one of her and Martin and is definitely more academically-inclined than her stepbrother, Diana can arguably be just as petty and immature. She just does so in a more dignified way.
- Overshadowed by Awesome: Almost 2/3 of the time she is overshadowed by Martin. This comes to a head in the third season finale.
- Promoted to Love Interest: Inverted—in the original comics, she and Martin were lovers who eventually got married, but in the TV-series, they're stepsiblings.
- Sibling Rivalry: To Martin, naturally. Not the pranking type, like Martin, but she more than makes up for it with put downs and petty snippiness.
A 200,000 year-old caveman and a friend and aide-de-camp to Martin and Diana who works at Torrington as a cook and a janitor.
- All Cavemen Were Neanderthals: Played With. He has the gorilla-like build of a stereotypical caveman and the Hulk Speak, but on several occasions, he's shown himself to be decently intelligent or at least resourceful. See Brains and Brawn.
- The Big Guy: Being a caveman, he's obviously the muscle of the trio.
- Brains and Brawn: Though not a genius, he's a lot smarter and more resourceful than you would expect from a caveman. His quick thinking comes in handy a lots of times, and one of the earlier episodes has him reading Goethe in a Freeze-Frame Bonus.
- Cloud Cuckoolander: Depending on the episode.
- Contemporary Caveman: He 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, serving as the team's tracker.
- Suddenly Voiced: In the original comic he's The Unintelligible to most (Martin and Diana have learned to understand him).
- Super Strength: Seriously, he punched through a stone wall. His brute strength proves useful when battling monstrous foes or breaking through barriers.
A tiny green alien who works for "The Center" as M.O.M.'s personal secretary.
- Ascended Extra: In-universe. At the beginning of the third season, he begins appearing as a regular at Torrington, after gaining a Full-Body Disguise that makes him look human (even though's he's about one foot and six inches tall).
- Beware the Nice Ones: He's a former space pirate overlord.
- Canon Foreigner: Created for the cartoon.
- Dark and Troubled Past: You can't be a former space pirate overlord, and not have this.
- Heroic Sacrifice: Subverted. Billy seemingly takes his own life to save the world from aliens taking over Earth, but as the trio and M.O.M. were grieving over their loss at the Center, he surprises them again by showing up, being thankful the ship's eject button worked.
- Lovable Coward: Not the type to run headlong into danger, but still a cool guy besides. Somewhat ironic given his past, but it could be explained by way of him suppressing his more aggressive traits.
- Really 700 Years Old: Looks like a child, and his human disguise is a teenager, but he's actually probably the oldest recurring character on the show. He was the alien at Roswell, and that was after years commanding an alien army - for all we know, Billy could be almost a hundred years old.
- Running Gag: He always appears out of nowhere on missions, scaring the main trio.
- Space Pirates: Once the leader of a vast army, but it became too much for him, and after years of behavioral adjustment at the Center, became the Billy we know and love.
- That Man Is Dead: As far as he's concerned, the space pirate overlord he once was no longer exist. Which his former followers don't take very well, and try to take over Earth as a result.
The multi-tasking director of "The Center".
- Beauty Mark: That is never consistent.
- Benevolent Boss: Holds a good relationship with the trio.
- Berserk Button: Every single time Martin breaks something in her office.
- Canon Foreigner: Created for the cartoon.
- Fun with Acronyms: According to Jerry in the crossover episode of Totally Spies, her name stands for "Mystery Organization Manager".
- Lethal Chef: It was once suggested that she take up cooking to alleviate her stress. The office looked like it had been attacked, and the "cookies" she baked looked like they were made of sludge.
- Meaningful Name: See "Team Mom" below.
- Mythology Gag: In the original comic the organization "Somewhere Else", effectively the prototype of "The Center", once had a director going by the codename "Dad". A female director.
- Omni Glot: Mentions in one episode that she aims to learn one alien language a day.
- Team Mom: She is Diana and Martin's boss, and seems to be close to them. Still, she got a bit heavy-handed when she once tried to play the role literally.
- The Stoic: Never panics, and almost never reacts to anything beyond either authoritative seriousness or vague amusement. Can lead to her being The Comically Serious.
- The Worf Effect: For being the Center's leader, she does get captured or possessed by many of the Monsters of the week whenever she's involved in a mission, although most of the time, she isn't expecting it.
- Work Aholic: Doesn't seem to have a life besides work, as she spends all her time at the Center.
Martin's rival. He does virtually everything that Martin himself can do (better, of course).
- The Ace: WAS this during his first two appearances.
- Canon Foreigner: Created for the cartoon.
- Doppelgänger: For lack of a better term, of Martin. Ironically enough, Martin's the only one who spots the similarity, while everyone is mostly focused on how great he is (of course, it doesn't stop M.O.M from punishing him for his screw-ups). As The Other Wiki states, he's like a Bizarro version of Martin: similar yet different.
- Greed: Occurs when he intentionally looked for a Leprechaun's charm, in the hope it would bring him good luck. Of course, it results in an Involuntary Transformation into a Leprechaun.
- Nice Guy: His characterization in his first few appearances, as part of his whole "like Martin but absolutely perfect" characterization. He loses this quite a bit after he gets Reassigned to Antarctica.
- Reassigned to Antarctica: Well, the Arctic, but that's it's the same basic thing.
- Recurring Character: Technically doesn't affect the story line in any way, but still is an example. Appears twice in Season 1, twice in season 2, with only one appearance in season 3.
- Spotlight-Stealing Squad: He's this in his first two appearances.
- Unknown Rival: Doesn't seem to be aware that Martin is jealous of him or even that he dislikes him in his first few appearances. He tends to respond to Martin's jeers with oblivious kindness. The rivalry becomes a lot more mutual after Marvin is Reassigned to Antarctica.
Martin's father and Diana's step-father.
- Invisible Parents: Not Gerard, per se, but we never see Diana's mother in the entire series.
- Like Father, Like Son: Subverted. Their interests are completely the total opposite of each other, which results in some conflict in the episodes he's with Martin, which is both of his appearances (his video call with Martin and Diana on the Christmas episode doesn't count).
- Parental Favoritism: Unintentional, but he seems closer to Diana and gets along better with her than he does with Martin.
- Science Vs Magic: Firmly on the science side, until he finds out about the Center.
- Skeptic No Longer: After the second time he gets involved in the paranormal (which involves him being possessed by a pre-historic worm), he is finally told of the Center.
- Strong Family Resemblance: Looks like an older Martin with facial hair.
Diana's best friend, whom Martin is always trying to get a date with. Generally a minor character in the various episodes' Framing Devices (though sometimes a Distressed Damsel when Torrington itself is under attack) but in any case still one of the very few notable (if not the only) recurring Torrington characters.
- Abhorrent Admirer: Considers Martin to be one, with a few exceptions.
- Depending On The Episode: How inseparable she and Diana are, whether or not she and Martin are also friends, how nice she is, etc.
- Jerkass: Depending on the episode. Sometimes she's outright nice, sometimes she's only a jerk when Martin bugs her too much, and sometimes she can be mean with little provocation. She gets generally meaner the more the series went on, however.
- Satellite Character: She's either Diana's friend or a girl Martin is hitting on, occasionally both. There's little to her character beyond these roles.
- Unknown Rival: Once was the indirect spark of yet another paranormal crisis when a girl jealous of her wished she could be more like her, and instead placed a dangerous curse upon herself. Jenni remained totally unaware of this - and in fact did not even mean to do the thing that made the girl jealous in the first place.
- Valley Girl: Talks like one. Sometimes acts like one, but not always.
This section is dedicated to a select number of creatures (or villains) that Martin and Co come across. The main ones posted here are those who at least appear in more than one episode, or have some significance to a character aside from one-shot monsters.
One of the few recurring creatures in the series. First appears in Crypt of the Djini, and later on in Return of the Djini
- Jackass Genie: In spades. Wish for a one-of-a-kind statue? She turns you into the statue.
- Our Genies Are Different: Trapped in a purple gemstone skull.
- Hoist by His Own Petard: Can't stop granting wishes, no matter what it is. One time though, another wish is reflected back at her, with similar results.
Gastromo was the first monster that M.O.M ever caught as an agent, and appears in Beast From Within.
- Puppeteer Parasite: Forcibly takes over his hosts, and through his touch, can also make other beings his slaves.
- Weaksauce Weakness: One linked to real life; slugs can't stand salt, and neither can Gastromo. Also, he consumes so much iron, the host becomes attracted to magnets.
- Big Eater: He makes his hosts into this to an extent, and he prefers to eat raw meat, since he requires a heck of a lot of iron.
Venoso was the first monster that Martin Mystery ever caught. Appears in Lovespell from the Underworld.
- Shapeshifting: Venoso's primary ability, is to change her shape. Like a normal chameleon, she can take the colour of her environment, making her invisible. She can also change her shape to look like whoever she wants to, like M.O.M, Vivian Michelle (Martin's first crush), and Martin himself.
- Overly Long Tongue: Of course, being a chameleon.
These villains and monsters are notable in the fact that their appearances invoke a two-part episode. There are four such two-part episodes in the series, each one headed by different villains.
Members of a Space Pirate army Billy once led back when he was Ganthar. They appear in "They Came From Outer Space".
- Curb-Stomp Battle: Martin and Co are absolutely no match for the Space Pirates, and Billy has to interfere to save them.
- Space Pirates: Obviously. They are a variant along the lines of Metroid Space Pirates.
- Kill It with Ice: Highly vulnerable to a fire-retardant like substance M.O.M was extracting from an insect earlier in the episode.
- Shapeshifting: Capable of this, and even morphing into one super-alien.
- Kill It with Fire: After their combined form is frozen, Ganthar tosses them into some computer equipment, making an explosion, destroying them.
- Planet Looters: Upon conquering a planet, they use up all available resources, and move on to other planets to conquer, leaving a line of barren worlds in their wake.
Appears in Season 2 Finale, They Came From The Gateway. Due to the two-part episode's nature, it is likely they intended for him to be the final monster of the series, until they continued Martin Mystery for a third season. He is the Guardian of the Gateway, a pool located in Tibet that is the source of all paranormal evil.
- Cliffhanger: His still being around at the end of the episode. As mentioned above, it is never mentioned what happened next during Season 3.
- The End... Or Is It?: At the end of the episode, he is seen in the now shut down Center, eying Martin and M.O.M's U-watches. This is after he was supposedly frozen outside the Gateway in Tibet.
- Omnicidal Maniac: As part of his nature, upon corrupting M.O.M with the Gateway's power, he turns all the creatures within it against the Earth, aiming to plunge the world into an eternal world of darkness.
- Our Liches Are Different: Appears to be an undead spirit, and is humanoid in appearance, matching the standard appearance of Lich.
- Ultimate Evil: While we do see him, he does fit the bill, considering he's the guardian to the Gateway of the Underworld, the source of all Paranormal evil.
Borack the Faceless One
An entity located in another dimension. He holds dominion over all shadows, and surfaces once every 1000 years to attempt a take-over of the world. Appears in the two-part episode, Day of the Shadows.
- Eldritch Abomination: Takes the form of a living shadow, and upon the day of his take-over, he is able to drag nearly every living being on the planet into his Shadow Dimension when there are shadows around them. Martin, Diana, Billy, and Kaitlin (Martin's temporary girlfriend), only escape this fate due to being around light on the initial night of Borack's awakening.
- Fate Worse than Death: While in his realm, everyone is being slowly turned into shadows themselves.
The series' official final villain. She is also one of the few human adversaries that Martin and Co have come up against. Appears in the third season finale, "It's Alive".
- Disproportionate Retribution: She was fired from the Center for not being a good enough agent. Her response? Spend years creating an Ultimate Monster to destroy the Center, and get revenge on M.O.M.
- Evil Mentor: Is one to Diana, who she deceives into joining her organization.
- Not So Different: Attempts to use this on Diana to get her to join her, after revealing her true nature.
- The Rival: Has a history with M.O.M.
- What Happened to the Mouse?: The last we see of her is being defeated by semi-reptilian Diana's newfound abilities and being told by her that she'll be locked in a closet until her "perfect monster" is taken care of. It's totally unknown what happened to her afterwards.