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."
Nerds Are Sexy: Not exactly a nerd, but Martin certainly thinks she is.
Not so Above It All: She likes to think she's the smart, mature one and that Martin is The Fool, but she spends quite a lot of time being as immature and petty as her brother - just in a more dignified way.
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.
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).
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 behavioural adjustment at the Center, became the Billy we know and love.
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.
Martin's rival. He does virtually everything that Martin himself can do (better, of course).
The Ace: WAS this during his first two appearances.
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.
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.
Voiced by: Michael Donovan
Martin's father and Diana's step-father.
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.
Parental Favoritism: Doesn't mean it, but he does like Diana a bit more than Martin. A bit of a Tear Jerker, considering Martin's his biological child.
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).
Science Vs Magic: Firmly on the science side, until he finds out about the Center.
Invisible Parents: Not Gerard, per se, but we never see Diana's mother in the entire series.
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.
Depending On The Episode: How inseparable she and Diana are, whether or not she and Martin are also friends, how nice she is, etc.
Jerk Ass: 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.
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.
Venasso was the first monster that Martin Mystery ever caught, sharing that trait with Venasso. Appears in Lovespell from the Underworld.
Shapeshifting: Venasso'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.
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.
Cliff Hanger: 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.