The series is about an orphan boy named Eric, with his best friend Trevor and his two female friends, Vana and Kitty, who are training to become sidekicks at the Academy for Aspiring Sidekicks in the city of Splittsboro. While dealing with intense sidekick training, Eric must also contend with his strict guardian Maxum Brain, his grumpy teacher Professor Pamplemoose, the evil Master XOX and keeping secret the disappearance of his superhero mentor Maxum Man from the city.
- Eric Needles (voiced by Miklos Perlus) - Eric was adopted as the sidekick to Maxum Man, the greatest superhero of all time (before he disappeared). He also has a huge crush on Vana, but he later seems to be more infatuated with Mandy Struction. He grew up in an orphanage because he has no known parents, although in the Funpak short: the Evil Trevor, Eric says that he once had a dad who spit on his face all the time, until he died. He was adopted by Maxum Brain. Eric always wears a T-shirt with a skull on it. He and Trevor always get into trouble and they don't have common sense. He and his friends are often forced to clean up the messes they caused. In the Funpak shorts, he was voiced by Peter Oldring.
- Trevor Troublemeyer (voiced by Christian Potenza) - Eric's best friend and partner-in-shenanigans, Trevor always ropes Eric into all sorts of hijinks. His father is Master XOX but he, along with everyone else, is completely unaware of this.
- Kitty Ko (voiced by Denise Oliver) - Despite being a bit strange, Kitty gives the rest of the group advice, and is somewhat a good hacker and fixer. She has a huge crush on Eric and has a bit of a stalker-like obsession towards him. She literally has two left feet revealed in "Trip Van Twinkle Toes". In the Funpak shorts, she was voiced by Stephanie Beard.
- Vana Glama (voiced by Stephanie Anne Mills) - Vana is a hyper-ambitious prima donna who is not easily impressed and is extremely shallow. She often tortures Eric and hates him for no apparent reason. Despite this, Eric has a huge crush on her, often oblivious to her selfish, snobby, and more-often-than-not sour-hearted character. In the Funpak shorts, she was also voiced by Stephanie Beard.
- Maxum Brain (voiced by Tony Daniels) - Eric's guardian due to the fact that Maxum Man is missing. He is a computer with lots of gadgets and acted as Maxum Man's assistant before his disappearance. He is strict about following rules and keeping the mansion clean and speaks with an East Indian accent.
- Maxum Man (voiced by Ron Pardo) - Eric's favorite superhero, before Eric became his current sidekick, with Superman-like powers. After he goes missing, Eric and Maxum Brain do everything for him to make it seem like he never left. It seems that Maxum Man always makes his own enemies purely by accident.
- Martin Troublemeyer (a.k.a. Master XOX) - Trevor's dad. All he wants is what is best for Trevor and for him to be a good boy, which is why Trevor can't stand him. He tries to stay calm because of his short temper. The only time Trevor likes him is when he gets angry. However, when Trevor is not looking, his dad becomes the super villain known as Master XOX. When he is Mr. Troublemeyer, he is voiced by Scott McCord and when he is Master XOX, he is voiced by Ron Rubin. He is the main antagonist of the series and he also Maxum Man's nemesis.
- Professor Pamplemoose - The cruel Headmaster at the Sidekick Academy and Eric's grumpy teacher, Pampelmoose is a strict disciplinarian who runs the school like it was prison camp. He has devoted his life to turning worthless students into worthless Sidekicks. He is voiced by Patrick McKenna. In the Funpak shorts, he was voiced by Alex Nussbaum.
- Golly Gee Kid - Golly Gee Kid was Maxum Man's sidekick in the old days, often seen in Maxum Man's Sidekick Academy training films. While a sidekick, it seemed that his primary job was to perform chores around the mansion. He is now the school's janitor. He is voiced by Ron Pederson.
- Mandy Struction - A student at the henchman school and Eric's new crush. She has seismic boots that can cause tremors in the floor whenever she stomps down. She is much taller than the other kids, suggesting she might be older than them. She also has a crush on Eric, which is why Kitty hates her. She appeared in the episodes "Match Dot Con", "Henchman for a Day", "Mandy-O and Eric-Et", "Oh Trevor, Where Art Thou?" and "Walter Ego Presents: Vapo House". She is voiced by Stacey DePass.
- Allan Amazing - Allan is a handsome student who is popular with the female students. Although he acts nice and sensible, he has an immense hatred for Eric and is a narcissist who will do anything to best anyone, especially Eric. He is voiced by Scott McCord.
- Kid Ruthless - A big tough student who seems to be a bully, as he tortures Eric for stealing his Sidekick Identity Name. He was popular with the students for his music. He was nice to Eric after Eric defeated him in a DJ Battle. He appeared in the episodes "Identity Crisis" and "Drop The Needles".
- Mayor Swift - The Senior Mayor of Splittsboro, all he wants to do is to see Maxum Man, which happened once in "Ain't No Party Like a Maxum Brain Party". He also has a hatred on a fired Super Hero, Static Clint, as shown in "The Spark is Gone". Swift also appeared in the Episode "The Maxum Switcheroo".
- Maxum Mom - Maxum Man's Mother, who has an intense dislike of Master XOX. She is tough like her son, and has visited and cared for Eric a couple of times in the Maxum Mansion. She has appeared in the episodes "Maxum Mom", "Shopping Spree", "Family Fun Day" and Eric poses as her in the Episode "The Maxum Switcheroo" to expose the fake Maxum Man.
- Maxum Mel - Maxum Man's older brother. He is tough like his brother and visited the Maxum Mansion one time. He has a sidekick named Mouse Boy. He appeared in "Of Mouse and Mel".
- Joshua Sideburns - A movie star with whom Kitty is obsessed. He visits Splittsboro to act in upcoming movies. He appeared in the episodes "Comic Book Zombies", "Teenage Mummies in Love" and "Maxum Method".
- Opossum Man - Another Superhero in Splittsboro who is a parody of Batman and always wants to make crime and criminals play dead like real opossums do. He also has a sidekick named Boy Vermin. He appears in "Opposum Man".
- Boy Vermin - Eric's old friend from the orphanage and Opossum Man's sidekick dressed as a rodent who is a parody of Robin and he always assists Opossum Man. His real name is Glenn. He appears in "Opposum Man". He is voiced by Deven Mack