Recurring
Phil Mygrave (voiced by Rob Paulsen) is Dick's stunt coordinator and brother and Riley and Todd's uncle. He is not very good with measurements, as he does not use proper units of measurements, rather just "tweaking the thingy on the whatchimacallit a smidge". He has also been married several times, and has poor advice for maintaining a relationship.
Agent B (voiced by Carolyn Seymour) is Agent K's mother and Todd and Riley's grandmother. She is the headmistress of the Royal Academy of Spies. She seems to not get along with K before the episode "London Calling", but in the episode they reconcile and B becomes a loving grandmother to Riley and Todd.
Agent G (voiced by Michael York) is Agent K's father and Todd and Riley's grandfather. He is the chief inventor of the Royal Academy of Spies.
Gordo Glideright (voiced by Bruce Campbell) is Dick's stunt rival. He is always trying to steal Dick's stunt secrets.
Dustin Dreamlake (voiced by Jason Marsden) is a parody of pop star Justin Timberlake and Riley's idol.
Ace Palmero (voiced by Dee Bradley Baker) – A news reporter who refers to himself as "I, Ace Palmero". A recurring gag involves only his profile being shown on TV, with the camera changing whenever he turns to face it.
Dr. Hans Herkmer (voiced by Jeff Bennett) is a scientist who works for the space program.
Amanda McMurphy (voiced by Candi Milo) is a hard-hitting investigator reporter who helped Riley with the school newspaper.
Fabian Le'Tool (voiced by Rob Paulsen) is a professional hairstylist.
Davey Hunkerhoff (voiced by Zac Efron) is a handsome lifeguard and parody of David Hasselhoff who Riley used to make Johnny Hitswell jealous in the episode "Davey Hunkerhoff".
Skye Blossoms (voiced by Tara Strong) is a prevalent replacement. A hippie who refuses to judge people and believes the answers to all of your questions are "what you feel the answer is."
Mr. Vanderbosh (voiced by Rob Paulsen) is Riley's stern teacher.
The Kelpmans (voiced by Chip Chinery and Mary Elizabeth McGlynn) are next-door neighbors to the Darings. They were once replaced on Halloween by the Zupecks.
Lady Lady (voiced by Grey DeLisle) is a professional wrestler who settled Tasumi and Riley's feud for Todd in "Best Friends For-Never?". Lady also appears as one of Dr. Scorpius' minions in "Irreplaceable".
Wrestler Announcer (voiced by Jim Cummings) announces the wrestling matches since Riley and Tasumi's end-of-friendship argument in "Best friends For-Never". He announced Lady Lady's Marriage to Canadian Knucklehead in "Serf's Up".
Celebrity Starr (voiced by Miley Cyrus in the first season and Jessica DiCicco in the second season) – This famous teen was first found replacing Shelton's imaginary girlfriend, Zelda because Shelton had defended Riley when Sierra put a love note in Riley's locker saying it was from Shelton. Riley felt poor and had Zelda replaced with Celebrity who liked nearsighted nerds. In the end, Todd turned her plan around and protected his friend. Celebrity also appears as one of Scorpius' minions in "Irreplaceable".
Zephremiah and Silent Joe (both voiced by Dee Bradley Baker) are twins who are friends of Todd's. They were both in Todd's boy choir. Zephremiah apparently likes sports. His twin brother, Silent Joe, is apparently sensitive and rarely talks.
Splatter Train is a fictional character in a recurring scary movie throughout the series.
Tiny Evil (voiced by Jason Marsden) – An enemy of Agent K.
Doctor Skorpius (voiced by Dave Wittenberg) – Archenemy of Agent K and the main antagonist of the series. He has a scorpion tail-shaped beard and is usually seen trying to take over the world in some way.
Goober (voiced by Dee Bradley Baker) and T-Bone (voiced by Jess Harnell) are radio DJs whom Todd idolizes.
Buck Spikes (voiced by John DiMaggio) is a tough baseball coach who replaced Todd and Riley's old coach, Pops. Spikes keeps pushing kids to the limit, and when they strike out, he cruelly throws them into a cage. Spikes also appears as one of Doctor Scorpius's minions in "Irreplaceable."
'Puter Dude 13 (voiced by Jeff Bennett) is a "cool and mysterious recluse" who runs the online interactive game Fleemster. He is secretly a nerd.
Heather Hartley is the "Her Girl" for Teen Swoon Magazine. She has an outrageous hairstyle and usually carries around her pet turtle on a leash.
Petrov (voiced by Dee Bradley Baker) is an enemy of Agent K.
Dr. Clonemaster is a rival of Agent K. He was seen only in the episode, "The Means Justify the Trend", in a flashback.
Dingo McGee (voiced by Jeff Bennett and Carlos Alazraqui) is the replacement sent by Fleemco when Todd got sick of an archaeologist treating them like babies on a field trip in the episode "Field Trippin'". He's an adventurous, usually reckless, explorer who plays the didgeridoo.
Robo Fleem SGX (voiced by Diedrich Bader) is a giant security robot that replaced the incompetent security guard at Todd's school in "The Insecurity Guard". However, the robot later went crazy and tried to destroy Riley when she tried to return it to Fleemco. In the last episode "Irreplaceable", Dr. Scorpius creates a second Robo Fleem SGX to destroy Todd's friends.
Mrs. Shusher (voiced by Tara Strong) is the shush-happy librarian at George Stapler Middle School. Todd got sick of her forcing everyone to be quiet, and later replaced her with a librarian who was the complete opposite of her. She returned later in the episode when her replacement was returned by Todd and Riley.
Gammazor, Mecha-Gammazor, and Grammazor – All rumored enemies of Tasumi. There are pictures of them on her "list" whenever she brings it out and threatens to put Riley on it.
Garth (voiced by Dee Bradley Baker) is a school janitor who is constantly being replaced by Todd and Riley when they need a quick replacement. Garth is not very enthusiastic about his job and is fully lazy. Garth also appears as one of Doctor Scorpius's minions in "Irreplaceable."
Mrs. Fragile (voiced by Kath Soucie) is a replacement who is very emotionally fragile, anxious and very easy to upset.