Master XOX is the main antagonist of Sidekick, because he is the most evil villain in Splitsboro and the arch-enemy of Maxum Man, who all the protagonists are affiliated with. His alter ego is Mr. Troublemeyer.
XOX is a pale while skinned man with black, scrawny hair and a short mustache. He has a big long nose and yellow eyes. He has dark purple lips and crooked yellow teeth. He wears a gray jacket and a black shirt underneath. He also has a butt shaped chin like his son. He wears gray pants and black boots.
As Mr. Troublemeyer, his appearance is less suspecting. His back is not hunched, his hair is placed on top of his head instead of the back, his teeth are perfectly white and his skin color is peach colored. His eyes are not beady and have no bags under them and lips are also normally colored. He wears a blue collared t-shirt along with a purple tie. He also wears jeans with a belt along with black boots.
Because XOX is an evil mastermind, he shares the stereotypical traits of a madman. He has a tendency to say 'evile' instead of evil. He demonstrates an incredible hatred towards Maxum Man, an example shown when in his room, Eric and Trevor see a bunch of Maxum Man posters with red Xs on them. As Martin Troublemeyer, Martin is perfectly sane and kind. He is very gentle towards the sidekicks, especially Trevor. However, he still has his hatred towards Maxum Man.
As Mr. Troublemeyer, he demonstrates ways to hide is identity as XOX by acting polite and nice towards the unsuspecting characters, which proves to be very effective. However, at times when he needs to vent off his anger, he changes into XOX and lets it all out. In Endless Summer, Eric and Trevor almost found out he was XOX during a road trip, but thought the two were different people.
- Like Supervillain, Like Son (First appearance)
- The Short List
- Maxum Mom
- Days of Golly
- A Monster Headache (Cameo)
- Virtual Mayhem
- Henchman for a Day
- Family Fun Day
- Internal Affairs
- Halloweenie (Mentioned, and Trevor dressed up in a costume of him, and As "XOXuman")
- Beneath the Missile-Toe
- Sleepless In Splitsboro (Dream; Only as Mr. Troublemeyer)
- Endless Summer
- Suit of Harms
- Four's a Crowd (briefly)
- Wrinkle Resistant
- Super Frenemies
- The Superest Day of the Year
- Fortress of Maxumtude
- Insane in the Cranial
- Eric-o and Mandy-et
- XOX is the only character to frequently visit both sides of Splitsboro.
- When XOX accidentally admits he went on a road trip to spend time with his son, but covers it up with another reason, it is shown that XOX does care for Trevor.
- He was the one responsible for mutilating Pamplemoose, as seen in the first Funpak short.