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.
- The Ace - Subverted. Coop could have easily been the smartest, most popular kid in town if he didn't have Kat to worry about.
- Butt Monkey: Thanks to Kat ruining his life.
- The Cassandra: Nobody (except Dennis and Fiona) ever believes him about Mr. Kat, even when trying to use photos
- Enemy Mine: Whenever a larger threat appears, such as Mr. Cheeks, Kat and Coop will work together.
- Friendly Enemy: To Kat. They are capable of getting along and working together for a common goal quite well. They even had a form of truce in the first Christmas special. However, these are rare occasions and they often go right back at each other's throats.
- Generation Xerox - Coop's life is awfully similar to his dad's childhood.
- Good Is Not Nice: Although he's the main protagonist of the show, Coop is sometimes a big giant asshole like much of the cast, especially to Mr. Kat and his sister.
- Heterosexual Life-Partners: With Dennis.
- The Hero: He's of the Loser Protagonist variant, but is nonetheless still the hero.
- Only Sane Man - He along his friend Dennis and later Fiona seem to be among the few well-adjusted folk in town.
- Pint-Sized Powerhouse: He is very skilled at fighting for a ten year old.
- The Scapegoat - If something goes wrong, no matter what the circumstances, people blame Coop.
- Will They or Won't They? - With Fiona
- They Do - They got married after the show.
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, this shows that Mr. Kat truly loves Millie with all of his heart.
- Alien Animals - Kat and the rest of his race happen to look and act like Earth cats, providing the plot of the show.
- Balloon Belly
- Kat gets one in "Fishy Frisky Business" after he gobbles up Colossal Coupon Mart's whole supply of Fishy Frisky Bits and ballooning bigger than the store.
- Big Bad Wannabe: He's the antagonist of the show, but becoming the Butt Monkey prevents him from getting close to the Big Bad status.
- Butt Monkey: He often gets his karma thanks to Coop.
- Deuteragonist: The antagonistic kind.
- Devil in Plain Sight - It's surprising how few people notice what he really is.
- The Dog Is an Alien: As Coop constantly tries to inform everyone.
- Enemy Mine - Whenever a larger threat appears, such as Mr. Cheeks, Kat and Coop will work together.
- Friendly Enemy - To Cooper. They are capable of getting along and working together for a common goal quite well. They even had a form of truce in the first Christmas special. However, these are rare occasions and they often go right back at each other's throats.
- Gadgeteer Genius: He can construct lethal weapons from household objects.
- Pint-Sized Powerhouse: Although he's the size of a cat, his physiology and skill make him very dangerous.
- Robotic Reveal - "Nip/Duck" reveals that Kat is actually a cyborg.
- Villainous Glutton - He really loves cat food.
- Woobie, Destroyer of Worlds: He is more than capable of destroying or enslaving the world, and has attempted to a few times. However many episodes indicate he really misses his family, and his "schemes" are just attempts to go home.
Voiced by: Kathleen BarrCoop's 8-year-old 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.
- Annoying Younger Sibling: She generally annoys Coop for a variety of reasons.
- Bitch in Sheep's Clothing: She acts all sweet and innocent but is a master manipulator, especially to Coop.
- Blind Without 'Em: Oh yes. She does not have good sight without her glasses
- Daddy's Girl: Mr. Burtonberger will often listen to Millie more than Coop.
- Hidden Depths: Surprisingly loves monster trucks as much as her brother does.
- Idiot Savant - Millie will usually turn out to be pretty skilled at whatever activity Burt tries to get her interested in - Whether it's building houses of cards or running the family store. But not at bowling, it turns out.
- Make Me Wanna Shout: She can scream really, really loud.
- Manipulative Bitch: Her profile states she is a master manipulator, and is always able to get her way, often using her deafening scream. Even Old Lady Munson gets along with her.
- Morality Pet - To Mr. Kat. The alien invader is explicitly shown to be very fond of her, and will even put himself in harm's way to protect her.
- Spoiled Brat - Throws tantrums if she doesn't get what she wants.
Voiced by: Cathy WeseluckCoop's 10-year-old 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.
- Absentee Actor - He does not appear or is mentioned in Let The Games Begin, Me-Oh Me-Oh Meow, Flu the Coop, One Big Happy Family, Fishy Frisky Business, Fat Kat, Kat Whisperer, Under Destruction, Hit The Road, You Kat See Me, Mind Games, and Strange Kat On A Train.
- Ambiguously Gay
- Butt Monkey
- Chaste Hero - Has little to no interest in romance, and reacts with terror when flirted with.
- Interestingly enough, he is in a relationship with Phoebe in the alternate timeline.
- Deadpan Snarker - Whenever Coop goes too far off the deep end.
- Generation Xerox - Dennis' life is awfully similar to his dad's childhood.
- The Lancer: To Coop, who is The Hero.
- Only Friend - The only kid willing to consistently stay friends with Coop despite his obsession with defeating Kat.
- Recurring Character
- Second Episode Introduction - He first appeared along with Lorne and Harley in Night of the Zombie Kat.
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.
- Genre Blind: Often why he's so oblivious to Kat's schemes.
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.
- The Munchausen - In "Down The Creek".
- Older Than She Looks - You'd hardly believe that was possible, but yeah, apparently she is.
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.
- Abhorrent Admirer - To Coop.
- Ambiguously Brown - Though her father seems to be Hispanic.
- Minor Major Character
- Stalker with a Crush - Phoebe can be less than subtle about her crush on Coop. After Fiona's appearance, she got worse.
- Yandere - She's pretty terrifying when she wants to be.
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.
- Action Girlfriend - She enjoys several sports and can easily keep up with Coop during fights against Kat.
- Not So Above It All - She can be as pig-headed and stubborn as Coop, which lead to a row between them.
- Promotion to Opening Titles - After her first debut.
- Will They or Won't They? - With Coop
- They Do - They got married after the show.