Guest
Sarah Gadon (season 1 episode: "The Wedding"), and later Paula Brancati (season 4 episode: "Truman's Last Chance"), as Victoria, aka. Vicki, Casey and Lizzie's cousin from Toronto. She and Casey appear to dislike each other, and things turn tense when Casey and Derek volunteer to organize the wedding reception of Vicki's mother, and Casey later finds out that Derek and Vicki made out when nobody else was aware. In Season 4 she and Casey appear to be on better terms, but she is revealed to be Truman's ex-girlfriend, and makes out with him while he is dating Casey. In the season 2 episode “Crushin’ the Coach” it is mentioned that she made out with the first boy Casey had a crush on.
Alex House as Trevor (season 2 episode: "The Bet"), a "punk" boy who goes to Derek and Casey's school. Derek bets Casey she can't get him to ask her out, and Casey takes the bet, soon developing a crush on him. She loses the bet after telling him about it after Derek interferes and makes her feel guilty for lying to him. She finds out Trevor was only hanging out with her because he had a crush on Emily, but the two stay friends.
Keir Gilchrist as Jamie (season 3 episodes: "It's Our Party" and "When Derek Met Sally"), Lizzie's best friend who harbors a crush on her.
Hannah Lochner as Michelle (season 3 episodes: "It's Our Party" and "Power Failure" and "Derek's School of Dating"), who first appears at Lizzie and Edwin's birthday party and becomes Edwin's girlfriend after kissing him there. Although she appears infrequently, she is often referenced.
Adam Butcher as Noel Covington (season 3 episodes: "Show-Off-Tune" and "Allergy Season"; and season 4 episode: "Just Friends"), an "artistic" boy who Casey befriends when he auditions for the lead role in a play on a dare, and, surprisingly, gets it, which leads him to co-star with Casey. He develops a crush on her. In the episode "Just Friends", Casey begins hanging out with him to prove to Derek that a guy and a girl can be friends, but it backfires when Noel thinks Casey asked him out on a date. In "Surprise" he does not appear, but is mentioned as a possible date to prom for Casey, but turns out to be going with one of Derek's backup choices.
John Nelles as Principal Frank Lassiter (season 2 episode: "Prank Wars"; season 3 episode: "Power Failure"; and season 4 episode: "Futuritis"), the new principal starting in season 2, who frequently ends up irritated by Derek, at one point nearly expelling him. In the film Life with Luca he has become a Customs and Border Protection officer, stating that he couldn't handle being a principal anymore after dealing with Derek.
Michael Kanevsky as Tinker Tomlin (season 2 episodes: "The Venturian Candidate" and "Prank Wars"; and season 3 episode: "Power Failure"), a timid boy at the school who follows authority and seems to have a crush on Casey.
Cameron Ansell as Teddy (season 3 episode: "Misadventures in Babysitting"; and season 4 episode: "Teddy's Back"), one of Edwin's friends, who is a prankster, but not as good as Derek.
Rick Roberts as Dennis McDonald (season 2 episode: "Dinner Guest"), Casey and Lizzie's biological father, Nora's ex-husband, who works as a corporate lawyer in New York.
Jennifer Wigmore as Abby (season 3 episode: "Adios, Derek"), Derek, Edwin and Marti's biological mother, George's ex-wife, who works as a marine biologist in Spain.
Barbara Gordon as Aunt Madge (season 3 episode: "A Very Derekus Christmas"), George's aunt.
Tara Manuel as Aunt Fiona (season 1 episode: "The Wedding"), Nora's sister, Casey and Lizzie's aunt, Victoria's mother, who marries another man to her daughter's disapproval.
