Cooper "Coop" Burtonburger
Voiced by: Erin Mathews
A 10 (later 11)-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, as in the end, Kat ends up destroying every trace of evidence. 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.
- 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 and almost everyone else treating him like trash.
- 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.
- Expy: Let's see: a kid who finds out that a certain newcomer is an alien, becomes Arch-enemies with them, tries to warn people about it, but they don't believe him probably because of ignorance and obliviousness. Plus he really isn't popular in school, so uh... if he ever puts on glasses and some geeky attributes he could pass off as Dib Membrane.
- 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.
- Heterosexual Life-Partners: With Dennis.
- The Hero: He's of the Loser Protagonist variant, but is nonetheless still the hero.
- Missing Mom: His mother is not mentioned at all and no clue is mentioned on her. He may have gotten her blue eyes from her given that besides them, he otherwise heavily resembles his dad.
- 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 almost always blame it on Coop.
- You Have to Believe Me!: Constantly, to his family, about Kat.
Mr. Kat
Voiced by: Kathleen Barr
A 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.
- Axe-Crazy: When provoked, which usually happens anytime he's conflicting with Coop.
- 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.
- Brilliant, but Lazy: A very intelligent being, but prefers to lounge around and do things for himself. Turns out this is a trait for his entire species; heavily smart, but too lazy to clean up after themselves, hence why they go from planet to planet.
- Butt-Monkey: He often gets his karma thanks to Coop.
- Cats Are Mean: He's a cat-like alien and the antagonist of the show so no surprise.
- Creepy Hairless Animal: Kat is this, which along with his black sclera and white pupils indicate he is no ordinary cat in the pilot episode. His whole race is this too, being a race of advanced cat-like aliens who desire to conquer and enslave Earth. Many characters frequently comment how ugly Kat looks, with one person even somehow mistaking him for an iguana.
- Cute Kitten: What Millie perceives Mr. Kat to be.
- 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.
- Expy: An alien who infiltrated earth, is confused by a household pet, gets adopted by a little girl who has quite a temper. While people take notice of their weird appearance like their black sclera they don’t pay mind to it. Their original name consist of a serial number, but they later get a new name after getting Happily Adopted. Are we talking about Kat or Stitch?
- Formally-Named Pet: He is called Mister Kat by Millie.
- 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.
- Hair-Trigger Temper: At times, it seems like it takes little to set Kat off.
- Heel–Face Turn: According to the show's creator, he became good friends with Coop after the series finale.
- 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.
- Temporary Bulk Change: He both puts on weight as fast as he can eat, and loses it as fast as everybody wishes they could.
- Trademark Favorite Food: Fishy Frisky Bits. Seriously, a large number of his evil plans revolve around acquiring them.
- 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.
Millie Burtonburger
Voiced by: Kathleen Barr
Coop'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.
- 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.
- Cloudcuckoolander: She is shown to have a mind that’s out there.
- Cute, but Cacophonic: She is adored by many for her cuteness, but has many cringe when she whines loudly.
- Daddy's Girl: Mr. Burtonburger will often listen to Millie more than Coop.
- Girly Girl with a Tomboy Streak: She definitely counts. While she’s shown to have most of the same traits that Phoebe has, such as playing dress-up and tea parties, but she’s also shown to be quite violent, and also likes monster trucks as much as Coop does.
- 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.
- Jerkass: She's constantly torturing her older brother just for laughs.
- 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.
- Missing Mom: Much like Coop, she doesn't speak of her mom. Though presumably, she got her looks from 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.
- Pet The Cat: To Mr. Kat, all the time, even when she's oblivious to the fact that he's doing something villainous.
- Spoiled Brat: Throws tantrums if she doesn't get what she wants.
- Super-Scream: She can scream really, really loud to the point of breaking glass and causing earthquakes.
- Wanting Is Better Than Having: A rather malicious take on this trope, as Millie will frequently want what Coop has....but only because she doesn't want Coop to have it. When Millie demanded the last of one box of cereal Coop was OK with it since he wanted some from another box, Millie demanded she have the last of that box instead.
Dennis
Voiced by: Cathy Weseluck
Coop'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.
- Ambiguously Gay: He has shown to have very little, if any interest, in women.
- However, in “Kat to the Future”, he is shown to be together with PHOEBE of all people! Then again, considering this takes place in an alternate timeline, perhaps this means alternate “sexuality.”
- Butt-Monkey: While he's not a scapegoat like his friend Coop, he's no better than his friend in this department.
- 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.
- Missing Mom: Subverted. Unlike Coop, his mother appears but is unnamed.
- Only Friend: He's Coop's only real friend and stands by him. As such, he often does his best to help Coop stop Kat from his schemes.
- Only Sane Man: Often tries to be this when things start escalating.
- Recurring Character
- Second Episode Introduction: He first appeared along with Lorne and Harley in Night of the Zombie Kat.
Burt Burtonburger
Voiced by: Trevor Devall
Father of Coop and Millie Burtonburger. He is constantly stressed out due to Coop's "misbehaving". He owns the House of Swap.
- Bumbling Dad: He isn't that stupid, but he's prone to screwing up and is always oblivious to Kat's schemes.
- Genre Blind: Often why he's so oblivious to Kat's schemes.
- Repetitive Name: Seriously? Burt Burtonburger?
- Sitcom Archnemesis: He's this to Old Lady Munson and seems to have been this since he was a kid.
Old Lady Munson
Voiced by: Linda Sorenson
A 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.
- Cranky Neighbor: She's very irritable and foul-tempered to say the least.
- The Dreaded: She is feared by everyone in town, except Millie.
- Fan Disservice: In Bringin' The Heat, she wears a yellow leopard mini bikini much to Burt's fear.
- Jerkass: Especially to the Burtonburgers. Subverted with Millie. This is because Coop's grandfather fell in love with another girl which makes Old Lady Munson jealous.
- Jerkass to One: She’s a Cranky Neighbor who Hates Everyone Equally, but she especially hates the Burtonburgers (minus Millie).
- Leitmotif: She is always accompanied by slow, intimidating theme music.
- The Münchausen: In "Down The Creek", when she tells plenty of fantastic stories to Millie. While Millie disregards them, the last shot is of pictures detailing this, implying Old Lady Munson was telling the truth.
- Older Than She Looks: You'd hardly believe that was possible, but it turns out she was already a crabby old lady when Coop's dad was a kid.
- Sadist Teacher: In "Coop D'Etat" she is the mean substitute teacher who tortures everyone in the class especially to Coop.
- Screw Politeness, I'm a Senior!: A scary old woman who's worst of a lot, unless you're Millie when she's around.
- Unstoppable Rage: She's extremely grumpy to everyone especially to Coop and Burt.
- Vague Age: God knows what her acutal age, but given that she was old when Coop’s own father was a boy, she has to be insanely elderly.
- Would Hurt a Child: Has actively tried to harm Coop on various occasions. In "Bootsville's Most Wanted", she joins an insane animal protection officer who tries to run over Coop and Dennis, only realizing it's gone too far when her garden gets destroyed during the chase.
Phoebe Del Oro
Voiced by: Tabitha St. Germain
A girl who has an unrequited crush on Coop and constantly annoys him. Phoebe clearly despises 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.
- Bit Character/Recurring Character
- Clingy Jealous Girl: When Fiona comes into the picture, Phoebe takes this trope up a notch. She will do various schemes to keep Fiona and Coop away from each other as much as possible, which includes smashing his birthday cake onto Fiona, trying to get her lost in a cave, and locking her in an old mill.
- Precocious Crush: She has a MAJOR crush on Coop.
- But in an alternate timeline in “Kat to the Future”, she has one on Dennis instead. However, unlike with Coop’s case, Dennis and her were actually DATING!
- 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, especially after Fiona's introduction.
Fiona
Voiced by: Chiara Zanni
Old Lady Munson's great great great great great grand neice, introduced in the second season. She is aware that Kat is an evil alien and Old Lady Munson tries to keep her and Coop apart.
- Action Girlfriend: She enjoys several sports and can easily keep up with Coop during fights against Kat.
- Affirmative Action Girl: The first real girl we see get into action consistently.
- Dumb Blonde: Inverted. She is shown to be clever at times, such as when she was grounded at home, she escaped by using a nail filer to pry open her locked bedroom window.
- Hair of Gold, Heart of Gold: She has long blonde hair, and is a very sweet and amiable person most of the time.
- Not So Above It All: While she's often the more level-headed and reasonable one, she can be as pig-headed and stubborn as Coop, which lead to a row between them.
- Only Sane Man: She becomes the second person other than Coop to believe of Kat's alien identity along with Dennis.
- Plucky Girl: She becomes the third person to become aware of Kat being an alien invader and thus helps Coop out in stopping Kat.
- Promotion to Opening Titles: After her first debut.
- Tomboyish Ponytail: Has her long hair in a ponytail, and she likes to play sports.
- Tomboy with a Girly Streak: She may be adventurous and a jock, but she can be mushy at times, and her casual attire involves a dress.
