Cooper "Coop" Burtonburger
Voiced by: Erin MathewsA 10-year-old boy who knows Kat is evil. He tries to stay out of trouble, but is often humiliated and injured when trying to prove Kat's true identity. He gets the blame for everything Kat ruins in the house. Coop sometimes thinks Mr. Kat plans is to take over the world, but Mr. Kat's plans are to return home or just to get food for himself. Coop may have misunderstand Mr. Kat most of the time. Coop's father thinks that Cooper is insane for thinking that Kat is an evil alien. Only Dennis, Coop's best friend, and later Fiona, as well, believe in him. The Burtonburger's neighbor, Old Lady Munson, hates Coop, but favors Millie, and she has a many greats-grandniece, Fiona, who Coop likes. Coop has brown hair and blue eyes.
Voiced by: Kathleen BarrA space alien who looks like a hairless Sphynx cat. Kat is adopted as a stray kitten by Millie Burtonburger. Blaming Coop for being stranded on Earth, Kat hates Coop though at times he and Coop get along, in-fact whenever trouble threatens both of them they work together as revealed in "Just Me and Glue." Kat tries to get home only to be interfered with Coop. There is a possibility that he likes Millie as he has been acting affectionate with her and even sleeps in bed with her. However, his affection for Millie may be fake. For example, in the episode "One Big, Happy Family" when Millie fell for a trap Mr. Kat made, Mr. Kat tried to save her. It's unknown whether if it was because he cared about her safety or she would just mess up his plan. In "Pet Peeved" Mr. Kat got jealous when Millie ignored him, it proves that Mr. Kat truly does like Millie.
Voiced by: Kathleen BarrCoop's younger sister and the "owner" of Kat. She loves Kat and is oblivious to the fact that he is an alien, and sometimes, she interrupts his evil plans. She can scream so loudly that the whole town can hear her. They call her the "master manipulator". Her father does everything she says. Millie has black hair, and she wears red glasses.
Voiced by: Cathy WeseluckCoop's best friend, he is the one of the only people that believes Coop, their birthdays are only one day apart, and they always wish for the same presents. Dennis has black hair and black eyes.
Voiced by: Trevor DevallFather of Coop and Millie Burtonburger. He is constantly stressed out due to Coop's "misbehaving". He owns the House of Swap. Burt has brown hair and black glasses.
Old Lady Munson
Voiced by: Linda SorensonA nosy old woman who is the next-door neighbor of the Burtonburgers. She is grumpy and mean to all the children and adults, though she favors Millie over Coop. She has a many greats-grand neice named Fiona, whom she tries to keep away from Coop.
Voiced by: Tabitha St. GermainA girl who has an unrequited crush on Coop and constantly annoys him. Phoebe clearly despise Millie (partially because Millie "owns" Kat and Phoebe owns Honeyfluff - an inferior, white-haired, obese cat). She has brown eyes, brown hair and brown skin.
Voiced by: Chiara ZanniOld Lady Munson's great great great great great grand neice. She is aware that Kat is an evil alien and Old Lady Munson tries to keep her and Coop apart. She has blonde hair and green eyes.