Cooper "Coop" Burtonburger
Erin Mathews. A motherless 10-year-old boy who thinks Kat is evil since they first met. 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's plan is to take over the world, but Mr. Kat's plans are to return home to his home planet in outer space. Coop may have misunderstood Mr. Kat most of the time. He always jumps to conclusions without knowing all the facts, due to the fact that he never takes chances and never bothers talking to Kat about the situation instead of fighting him.) The irony about Coop is that he always comes up with plans to get rid of Kat, which only cause Kat to stay. Coop's father, Mr. Burtonburger, thinks that Coop is insane for thinking that Kat is an evil alien. Only Dennis, Coop's best friend, believes him. The Burtonburgers' neighbor, Old Lady Munson, hates Coop, but favors Millie. Coop has brown hair and blue eyes. Nobody knows what happened to Coop and Millie's mom who could either be deceased or divorced.
"Kat" or "Mr.Kat"
Kathleen Barr. A Space Alien that looks like a Sphinx cat. He's adopted by Millie as a stray cat. Blaming Coop for his being stranded on Earth, Kat has made himself Coop's sworn enemy, 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 repeatedly tries to get home, only to be foiled by Coop. In "Nip/Duck" he turns out to be part cyborg. It is very, very clear that Kat loves Millie in the episode "Pet Peeved," as he constantly calls for Millie's attention at the start of the episode, then runs away because Millie had found a new playmate. In the episode "One Big, Happy Family," when Millie falls for a trap Mr. Kat had planted for Coop, it is unknown whether Mr. Kat tries to save her or not.
In "The Kitty Vanishes," it is revealed that he has a girlfriend who looks similar to him, but with smaller and more feminine eyes.
In "UF Float," inside his luggage, Mr. Kat has a picture of Millie.
Kat's main goal is often presented as simply trying to get home, although in one episode, "Fat Kat," it is revealed that he was sent to Earth to gather "Fishy Frisky Bits," a type of cat food, and transport the substance back to his home world, the planet Catnip, where the promised title of hero would be waiting for him. It is also revealed that he has been doing a poor job of it so far. Another episode, a Christmas special, reveals how much he misses the family he left behind on his home planet.
Mildred "Millie" Burtonburger
Voiced by Kathleen Barr. Coop's younger sister; a spoiled and bratty girl and the "owner" of Kat, for whom she loves very deeply. Mr. Burtonburger, her father, always does whatever Millie says, mainly because she throws extreme tantrums when she fails to get her way. Because she can easily manipulate people into giving her what she wants, her talent earns her the neighborhood nickname "The Master Manipulator." Like several characters, she blames Coop for everything that happens. Millie has long, dark hair tied up in a ponytail with a magenta-colored bow, freckles and buck teeth, and wears red-framed eye-glasses. She is a "Greeny Girl;" the Greeny Girls are a parody of the Girl Scouts. In the episode "Pet Peeved," Millie meets a girl who is exactly the same as her, only she looks a little different. Millie's hair goes down to her feet.
Voiced by Cathy Weseluck. Coop's Asian American best friend and the only person that believes Coop. He helps Coop whenever he tries to stop Kat by contacting each other on Walkie-Talkies. Their birthdays are only one day apart and they always wish for the same presents. Dennis has black hair and black eyes but unlike his father, he does not have an Asian accent.
Old lady Munson's niece. A girl that Coop has a crush on. She is the only one besides Coop and Dennis that knows Kat is not a normal cat. She sometimes visits Munson, and seems to like Coop, but Munson won't let them be togther. Voiced by Hilary Duff.
Fiona Munson (Voiced by Chiara Zanni) appears in season 2 in a episode called "Something about Fiona", She is currently in Juvey, She also appears in the new intro so she will most likely appear in some more episodes
She is very good at skate boarding and is proud of it, she also appears to be good at soccer and has an intrest in aliens and enjoys playing pranks on people
Voiced by Trevor Devall. The single father of Coop and Millie Burtonburger, who is constantly stressed out due to Coop's "misbehaving." He owns the House of Swap. Burt has brown hair and wears black-framed eye-glasses. He sometimes makes unpleasant-tasting vegetarian smoothies which no one can stand to drink. He seems to be strongly in favor of doing things yourself and living in harmony with nature, in contrast with Henry, who is very technology-savvy and uses gadgets whenever he can. Burt seems to hate that Coop is pulling his leg too much with jokes and gags. Nobody knows what happened to Burt's wife, the Burtonburger children's mother, who could have either divorced him or died.
Old Lady Munson
Voiced by Linda Sorenson. A nosy old woman who is the next-door neighbor of the Burtonburgers. She is grumpy, mean, and very selfish to all the children and adults, though she favors Millie over Coop. When someone (Kat very often) causes trouble for her and Coop ends up with the blame, she yells, "BURTONBURGER!" She is often seen carrying a blue umbrella with her, for no reason that the stories make apparent. She also owns a small dog named 'Growler', of whom she dresses in a fleece .
Voiced by Tabitha St. Germain. A girl who has a crush on Coop and constantly annoys him. Phoebe clearly despises Millie, partially because Millie "owns" Kat and Phoebe owns Honeyfluff, a white-haired, obese cat with a pink bow on the end of her tail who is shown, in one of the short episodes, to have an attraction to Kat. She has brown eyes, russet brown hair and light brown skin.
Voiced by Vincent Tong. Dennis' Asian American father. They have the same head shape, but Henry is almost bald. He and Burt see everything as a competition. Henry uses devices all the time.
- Kat's Girlfriend, who only appears in "The Kitty Vanishes," shares a romantic relationship with Kat, her boyfriend. (The provider of her character voice is not known.) It was rumored, from the date "The Kitty Vanishes" was first transmitted, that she would return in the 2nd season of Kid vs. Kat as a main character
- Tuttankitty: Not too much is known about him, but in legend he is said to be an evil tyrant, and Kat's ancestor. He draws bad luck with him.