Robbie Rotten is the main villain in LazyTown, played by Icelandic actor Stefán Karl Stefánsson.
The character normally wears a two-piece suit of maroon and purple with gold pinstripes, formal shoes, a navy blue shirt with cuffs and cufflinks and a gold pocketwatch. His hair is black and extremely slicked, as are his eyebrows. Robbie's eyes are seen to be grey in the series, however, some official merchandise has depicted him with brown eyes. He is the tallest character in the series, and is very thin.
Robbie is the Master of Disguises. While it's quite obvious for the audience it's him, for the LazyTown residents it is quite difficult to catch him. The only way for them to know it was him, is at the last moment when parts of or the entire disguise fails. He has some sort of an Automatic Wardrobe machine, which usually has the right disguise for him.
- The Mailbox: The LazyTown Mailbox, which is usually used by Robbie as hiding spot.
- Rob U. Blind: A traveling salesman who sells a machine for cleaning shoes.
- Lazycus: A SuperHero who attemped fill Sportacus place while he has troubles with his feet.
- Sherlock Foams: A Detective who accused Sportacus of stealing Bessie's Birthday Cake
- Sportacus: Robbie dressed as Sportacus, because the kids always listen to Sportacus, so they finally listen to him.
- The Birthday Fairy: The "traditional" fairy of birthdays, who comes to give Ziggy a present.
- Scotty the Scoutmaster: A Scoutmaster who comes to teach how be "Lazy Scouts"
- Dr. Rottenstein: A Doctor who declared an epidemic disease with the origins in the SportsCandy
- Rottenbeard: An old Pirate who attacked LazyTown once, who returns searching for an old treasure
- The Dinosaur: A purple dinosaur which tries to scare Sportacus. Off the record, Robbie cannot see anything using this costume.
- Sir Rottenham of Robbington: A knight who comes for the new prince of LazyTown, Stingy
- The Rotten Kid: A Western Thief who had a duel with Sportacus
- The gorilla suit: A Gorilla which tries to scare the kids and make them stop playing the Jungle Animals
- Miss Roberta (character): A manners expert, who comes to teach the kids the opposite way
- Fordmil Meansbad: The new Mayor of the town, who wins in a pretty weird election. The name comes from altering Mayor Meanswell's name.
- Santa Claus: A Santa which comes in a "Grinch" mode.
- The purple cow: A Cow just like the ones from his billboard
- Giant fruit and veg: 3 giant veggies and fruits, which tries to scare the kids from eating healthy
- Zobby: An Alien which comes to scare the kids, but ultimately befriends Ziggy.
- Johnny B. Badd: The [[Mayor's favorite singer, who comes to play a concert in LazyTown; in exchange the LazyTown residents have to do everything he says.
- A purple Bush: A bush for camouflage.
- The Trash Man: A trash man who collects all the trash and blasts it over LazyTown
- Mayor Meanswell: Robbie dressed as Mayor Meanswell, in order to make the rules in the way he wants.
- Bob Wire: A Computer Technician contracted by the Mayor in order to modernise the town.
- Big Bad Wolf: The Main Villian of the Three Piggies Story, who comes back to change the original story.
Robbie Rotten is also an Inventor and most of his inventions are made from scratch, but he has several ways to obtain a weird machine: his "Inventor Microwave" (A Microwave which make inventions) or simple purchase by phone, using a incredibly fast system.
- Candy Faker Maker 3000: A device to make candy appear as fruit.
- Memory Zapper 3000: Device created to erase Sportacus' memory and make him forget he was a hero.
- The Lazy Ball: A Purple Ball which produce awful sounds, avoiding the owner to sleep.
- Soccerbot: A Robot bought to beat Sportacus in a soccer game.
- Rottenella: A doll from a Music Box, enlarged just to make her participate in a dance contest in order to beat Stephanie.
- Littlizer 3000: Device used to shrink Sportacus to the age of 10.
- Trash Blaster: A Trash Truck with a Cannon attached, which sent away piles of garbage.