Master XOX is the main antagonist of Sidekick. He is the evilest villain in Splitsboro and the arch-enemy of Maxum Man, who all the protagonists are affiliated with. His alter ego is Mr. Troublemeyer.
As Mr. Troublemeyer, XOX's appearance is generic and unsuspecting. He is a nice suburban dad with peach colored skin, neatly combed black hair, and a shortcut mustache. he wears a blue collared t-shirt with his purple tie, complete with blue jeans, a black belt with a gold buckle, and black boots.
Master XOX has the same body frame as his alter ego, albeit he is more hunched over and the hair on the top of his head falls more on the back of it. His main differences in appearance come from a different color pallet and clothing attire. He is a pale-skinned man with dark black, scrawny hair and a short mustache. He has a big, long nose, beady yellow eyes, dark purple lips, and crooked yellow teeth. Another facial feature is his butt-shaped chin. XOX's attire consists of a gray jacket and a black shirt underneath. He also wears gray pants and 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, and an example is shown in of his room; Eric and Trevor see a bunch of Maxum Man posters with red X-marks on them. In XOX to be You, he says "pumpkin pie" whenever something goes wrong, and desires to defeat Eric for reasons unknown during the same episode.
As Mr. Troublemeyer, he demonstrates ways to hide his identity as XOX by acting polite and nice towards unsuspecting characters, which proves to be very effective. He is perfectly sane and kind in this form, and shows this by being very gentle towards the sidekicks, especially Trevor. However, he still has his hatred towards Maxum Man. Whenever he needs to vent off his anger, he makes the excuse of leaving the area for a bit, but then changes into XOX when no one is looking and lets it all out. In Endless Summer, Eric almost found out that he was XOX during a road trip, but everyone else otherwise believes that the two were completely 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, combined with Maxum Man As "XOXuman")
- Beneath the Missile-Toe
- Sleepless In Splitsboro (Dream; As Mr. Troublemeyer only)
- 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
- Parent Teacher Night of Doom (As Mr. Troublemeyer only)
- 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. This proves that XOX does care for Trevor, but prefers to hide it.
- He was the one responsible for mutilating Pamplemoose, as seen in the first Funpak short.
- When he is XOX, he is voiced by Ron Rubin. When he is Mr. Troublemeyer, he is voiced by Scott McCord, who also voices Owen and Trent from "Total Drama", Yang from "Yin Yang Yo!", McGee from "Camp Lakebottom" and Jake from "Paw Patrol".