- Krypto: The hero and protagonist of the show, formerly Superman 's pet on Krypton before it was destroyed. He has all the powers of Superman, though some are heightened due to being a dog, such as his super hearing and sense of smell. Krypto also shares Superman's moral values and feelings, which has been used as a weakness against him in some cases. This version of Krypto has a slightly different origin story than the comic series. Krypto is voiced by: Samuel Vincent.
- Ace the Bat-Hound: Batman's pet (though Ace says he is Batman's partner) Ace fights crime utilizing various detective skills and gadgetry built into his collar (an analogue of Batman's ); he also uses a rocket sled for traveling long distances. Ace wears a dark colored cape and cowl similar to Batman's, and (like Batman) possesses a stoic personality, however Ace does occasionally work alongside Krypto and considers him an ally. Ace is voiced by: Scott McNeil.
- Streaky the Supercat: An orange tabby cat who lives next door to Krypto and Kevin's family. In episode 3, "The Streaky Story", Streaky gained superpowers by a duplicating ray that was aimed at Krypto, but reflected off of him and hit Streaky instead; Streaky later found that he had superpowers similar to Krypto's. Streaky belongs to a girl named Andrea. Unlike Krypto, Streaky is less serious about his superhero activities, and sometimes needs prodding to perform heroic deeds, but usually is dependable. Streaky is voiced by: Brian Drummond.
- Kevin Whitney: Kevin is the young boy who Krypto lives with in the series, with Superman's permission. Kevin enjoys Krypto's company greatly, and is able to communicate with him and the other animals in the series thanks to a universal translator device. Kevin has a bratty cousin named Bailey who finds out Krypto's secret, but fortunately he's such a notorious liar that no adult believes him. Kevin also has a two-year-old sister named Melanie, who refers to Krypto as "Kippo". Kevin is voiced by: Alberto Ghisi.
- Andrea: Kevin's next door neighbor, and the owner of Streaky. Andrea enjoys making Streaky play dress-up, but wasn't aware of his (or Krypto's) superheroic identity until later in the series, when she accidentally stumbles upon Krypto's spaceship. She loves to dress up Streaky in different clothes and treat him like a baby. Andrea is voiced by Tabitha St. Germain.
A superpowered group of who each possess a unique superpower, and fight crime across the galaxy. Krypto joined the Dog Stars by helping the group out on a mission. Ace the Bathound and Streaky the Supercat helped them out on different occasions. The members of the Dog Star Patrol include:
- Brainy Barker: (voiced by Ellen Kennedy): A purple Afghan Setter in appearance (although she refers to herself as an Afghan in the episode "Meet the Dog Stars"), she is the leader of the Dog Star Patrol. Brainy Barker possesses powers, allowing her to read minds, project forcefields, and levitate objects.
- Mammoth Mutt: A pink chihuahua with the ability to inflate her body to an enormous ball-shaped size, allowing her to use her size to attack foes. She can also expand other parts of her body. Voice: Kelly Sheridan
- Bull Dog: A bulldog with stereotype British gestures and two horns that can be used for attacking foes and breaking objects. Voiced by Michael Dobson
- Paw Pooch: A dog with the ability to grow/sprout multiple limbs, allowing him to dig and run quickly. Voice is by Dale Wilson
- Tail Terrier: A terrier with the ability to stretch his tail and use it like a lasso. Also talks with an accent, he is voiced by Peter Kelamis
- Tusky Husky: A husky with a stereotype French-Canadian accent (provided by voice actor Terry Klassen) and a giant front tooth which he can use as a drill.
- Hot Dog: A dachshund with the ability to generate tremendous heat and breathe fire. Voice actor is Trevor Devall.
- Stretch-O-Mutt: Buddy, a guard dog at STAR Labs, fell into a vat of chemicals and developed shape-changing powers similar to Plastic Man. Sometimes he can be too silly for his own good. Voiced by: Lee Tockar.
- Drooly: A sheepdog who can use his drool as a weapon in various ways, such as grabbing objects, creating bubbles, and use them as projectiles. Voiced by Ty Olsson.
- Thundermutt : An egotistical canine actor who plays a heroic canine in the movies. He is somewhat jealous of Krypto and Ace, but though cowardly, mostly good at heart.
- Jimmy the Rat : A rat who often serves as an informant for Krypto and Ace (or even a villain if he is bribed, or tortured hard enough.
- Robbie the Robin: Robbie is a robin who was saved after he witnessed an encounter between Ace and The Joker's hyenas. After that, he decides to be Ace's sidekick, much to Ace's dismay. Robbie was based on Robin .
- The Supercat Fan Club: Led by Streaky's nephew, Squeaky, they are a group of Supercat fans.
- Melanie (voiced by Tabitha St. Germain): Kevin's baby sister. She knows that Krypto ("Kippo" as she puts it) is Superdog, but being a baby, her parents don't take her seriously.
- Bailey : Kevin's bratty cousin. He too discovered Krypto's secret and has unsuccessfully tried to reveal it.
- Smokey: A Dalmatian who appears in Old Dog, New Tricks. He believes that the firemen are going to replace him with a new puppy. So Streaky paints spots on Krypto to make him Smokey's super-double to prove that he's still up to the job.
- Blaze: The new Dalmatian puppy in Old Dog, New Tricks. Streaky thought he was going to be Smokey's replacement. It turns out he was Smokey's firedog trainee.
- Mechanikat: The main villain of the series, Mechanikat is a feline who constantly plots to conquer Earth. He usually keeps a bit of kryptonite on hand to use against Krypto, or to power up his machines with so they can affect Krypto. He is also a member of the Villains Club. Voiced by Mark Oliver
- Snooky Wookums: Mechanikat's and secret agent, Snooky is an evil mastermind kitten who uses his cuteness and smarts to cause all sorts of trouble for Krypto and the Dog Star Patrol. He later became part of the Supercat Fan Club.
- Delilah: Another feline agent of Mechanikat. Delilah's outfit changes in season 2, adding on violet-colored plates of armor in various areas on her body.
- Ignatius: The pet of Superman's archenemy, Lex Luthor. Like Luthor, Ignatius is very intelligent, vain, and is morally ambivalent about making others suffer for personal gain; however, he is effete, selfish, and tends to behave in a much less dignified manner than Luthor. He is also more prone to engaging in frivolous (and dangerous) personal whims, such as employing a growth ray to enlarge a caterpillar for his supper, or using a to harvest one of Krypto's hairs as part of a scheme to grow himself a lovely fur coat. Notable is that on occasion he shows basic morals and honor.
- The Joker's Hyenas : A pair of Hyenas who serve the Joker. Some confusion about which is Bud and which is Lou can be found, considering that both look almost exactly the same other than their collars. Bud wears a purple collar, and Lou wears a green collar. Their names are a reference to comedians Abbott and Costello.
- Isis: A female cat who works with Catwoman. Isis, like her mistress, is seductive, and also a master thief. Streaky was initially smitten with her.
- The Bad News Birds: Three birds who work with the Penguin (who, like the Joker and Catwoman, never actually appears in the show) and are usually up to some elaborate caper. They are Artie the Puffin, Griff the Vulture, and Waddles the Penguin.
- Dogwood: A half-dog, half-plant hybrid who can turn any plant or tree into living beings. Although his powers are reminiscent of Poison Ivy 's, he doesn't seem to be associated with her. He bears a striking resemblance to another cartoon canine.
- Mertin the Magnificent: A magician's rabbit that carries around a magic wand he stole from his master. Mertin is a master of magic himself. His only real intentions are stealing carrots, but he can hold his own against Krypto and Ace if confronted by them.
- Super Flea: A Kryptonian flea who stowed away on Krypto's ship. Like all Kryptonion creatures it gained a multitude of powers under Earth's yellow sun. it only appeared in the episode Super Flea
- The Junkyard Dogs : A group of ne'er-do-well strays led by Muttsy, a street-wise but greedy mutt. One member, Beazle, looks similar to with a . At best small-time hoods, they do periodically menace the heroes, and once took advantage of Krypto when he lost his memory due to kryptonite exposure.
- Krypto's Tail: Once when Krypto was exposed to red kryptonite his tail came to life and was separated from him and caused havoc.
- Blackbeak and the pie-rats: A thorn on Scretch-O-Mutt's side. Cracker and Cheese thieves. Blackbeak is a parrot while the others are rats.
- Barrump Barrump: A sinister space monkey who pulls pranks all around the galaxy. In one episode, he steals a device that freezes time so that he can pull pranks on everyone on Earth. He is known as the "Primate Prankster", and is on of the galaxy's most dangerous criminals.