Recurring characters

• Roz and Tom Patterson (voiced by Sally Cahill and Tony Rosato) are the parents of Warren and Tracy. Tom works as a bus driver.
• Aunt Dolores (voiced by Jayne Eastwood) is Warren and Tracy's aunt, a crabby and stuck-up but serious and well-meaning police officer and the older sister of Tom Patterson. She has a vendetta against the monster, unaware of it being her nephew.
• Billy Castleman (voiced by Daniel DeSanto) is an overweight, mean school bully who often ridicules Warren. He's hellbent on proving Warren as the monster, but is unable to do so.
• Gorgool (voiced by Len Carlson) is a sorcerer and the would-be ruler of Fenrath, who is trapped in a ball. He intends to steal the Jewel of Fenrath and the Book of Spells with his servant to free himself from the ball.
• The Servant (voiced by Howard Jerome) is the dim-witted servant of Gorgool. He carries Gorgool around and follows Gorgool's every command. His real name is never mentioned.
• Wesley Whiffington III (voiced by Christopher Ralph) is a child who appears in Season Two. He is spoiled and never gets things his way. He is also interested in adult projects such as business and moviemaking.
• Miranda Bell (voiced by Alyson Court) is a girl who said she became a scientist at age six and says that her brain weighs eight pounds.
• Morgool (voiced by Len Carlson) is Gorgool's twin brother and the ruler of Fenrath. He is not trapped within a ball and is far more dangerous and evil than his twin. It is believed that Morgool is unable to control anyone who is superior to him, such as the monster, and his jealousy and greed for the throne resulted in Gorgool's confinement to the glass ball.
• Ms. Gish (voiced by Barbara Franklin) is the teacher who is involved with most episodes in Season One.
• Kragon (voiced by George Buza) is a guard working for Morgool in Fenrath. He always helps his master in his schemes, mainly trying to capture the monster. Kragon's home region is Gorem Range.
• Red (voiced by Megan Fahlenbock) is an orphan from Fenrath who becomes friends with Warren and Tracy. She is always up for adventure and kicking bad-guy butt.
• Freddie Mitchell and Connor (both voiced by Susan Roman) are friends of Warren's. Freddie loves to tell jokes and play soccer. Connor's hobbies include pogo jumping, playing the piano, and video games.
• Keebo (voiced by Hadley Kay) is a shapeshifting creature from Fenrath.
• Dr. Malarmar (voiced by Adrian Truss) is a criminal and master thief who loves to dress up to fool people. He first appears as a fake ghost hunter and expert for the occult on "Haunted House" where the kids and Johnny can thwart his plans to rob the Pattersons. He later returns in "Thin Ice" as a Russian ice skating coach where he attempts to steal diamond-clad ice skates. He again returns in "Warren's Nightmare" where he wants to get revenge on the kids in Warren's bad dream.
• Dr. Fossel (voice by Michael Fawkes) is an archeologist and historian who works at the Pickford museum. He buys the magic book in the episode "Fossel Remains" (after Roz unknowingly had sold the book at a local flea market) and discovers its magical properties. He causes havoc after reading some of the spells out loud. The kids later befriend him and explain the situation to him, which causes the doctor to give them the book back. Dr. Fossel also learns that Warren is the monster, meaning that he's one of the few people who are aware about Warren's secret. The doctor later invites the kids to an archeological exhumation in the episode "The Big Dig". Dr. Fossel appears in many episodes, but he remains mostly a background character. The episode "Warren's Nightmare" implies that doctor Fossel is also a biologist, though it isn't clear if this is correct or not, given that Dr. Fossel seems surprised in Warren's dream that the boy is the monster, even though the real Dr. Fossel is fully aware of Warren's secret (it's also implausible that Dr. Fossel would want to torture his friend with sadistic experiments).

Pilot (1996)

Season 1 (1999)

Season 2 (2000)

Season 3 (2003)