Robotboy is the latest creation of the world-renowned scientist Professor Moshimo. Due to fears that Robotboy would be stolen by his arch-enemy Dr. Kamikazi and his main henchman Constantine to be used to take over the world, Professor Moshimo entrusts Robotboy to 10-year-old Tommy Turnbull, his biggest fan. While being protected by Tommy and his two friends Lola and Gus, or "G-Man" as he calls himself, Robotboy learns how to behave and act as if he were a real boy while occasionally battling Dr. Kamikazi and Constantine who sought out to capture him.
- Robotboy - The robot title character of the series is best friends with Tommy Turnbull. Robotboy has three modes: deactivated, activated, and superactivated.
- Tommy Turnbull - A blond, square-headed boy and Robotboy's 10-year-old owner and human best friend. He is smart, responsible, sensitive, likable and acts as a mentor to Robotboy. Tommy tries to teach Robotboy the ways of the world through his eyes
- Augustus "Gus"/"G-Man" Turner - Gus is overweight, weak-willed and selfish. He loves playing pranks on people (but most of the time they go wrong) and boasting about many skills that he claims to have... but obviously doesn't. Despite all that, Gus is one of Tommy’s best friends." .
- Lola Mbola - 10-year-old Lola is the confident and smart daughter of a rich African ambassador. She can fly jet planes and control speedboats despite her age, and was secretly in love with Tommy (but would be devastated if he ever found out)."
- Professor Moshimo - Professor Moshimo is the creator of Robotboy, Robotgirl and Robotbird (also Protoboy and Robotman). He received a letter from Tommy Turnbull about him looking forward to his latest invention. As a result, Professor Moshimo sent him Robotboy to take care of. The Professor has a Japanese fiancee named Miumiu, who also acts as his assistant. He also created Protoboy, Robotman and Robotgirl. His former rivals are Dr. Kamikazi and Bjorn Bjornson. He is voiced by Togo Igawa.
- Miumiu - Professor Moshimo's fiancée and lab assistant. She is typically depicted as silent, however she speaks for the first time in the episode "Grow No-Mo". While she rarely talks, she does seem to be annoyed with Moshimo's many hobbies. She is voiced by Laurence Bouvard.
- Robotgirl - A super fighting robot just like Robotboy, who was introduced in the episode of the same name "Robot Girl" and her last appearance was in "The Return of Robotgirl". She quickly made good friends with Robotboy and was only seen superactivated twice, both times in episode "Robotgirl". In the first place, she had to be convinced by Moshimo to use her ultimate powers, otherwise she would never see her friends again. In "The return of Robotgirl" she was brought back to Tommy with a distress message by Moshimo, she could not be activated until she was repaired by Tommy and she suffered from a major memory loss, but was convinced by Lola to use her emotions in favour of the kids' team. In the end she was defeated in a fight against Protoboy and then was saved by Robotboy.
- The Human Fist -
- Dr. Kamikazi - A small old man with thick glasses and a suit/bathrobe combo who is a self-proclaimed evil genius and the main antagonist of the show. He wishes to capture Robotboy to create a template for an army of super robots, with which he will be able to achieve his dastardly goal of world domination. Most of his henchmen have a large letter K on their uniforms, to show that they are on Dr. Kamikazi's side. He has had many pets, ranging from snakes to cats, but they usually hurt him.
- Constantine - A fat hunchbacked hitman of Dr. Kamikazi who also is a master of sumo wrestling (though it is unknown if he has ever been a yokozuna, or "grand champion"). He is a sensitive and none too threatening sidekick to Dr. Kamikazi who mainly acts as his muscle power as well as his assistant. Constantine is also a refined chef, being able to create dishes ranging from Sashimi to tapas. It was mentioned in some episodes that Constantine used to be an orphan before working for Kamikazi. Despite looking like a Japanese version of a hitman, Constantine is actually Spanish, suggesting that his outfit is simply a uniform assigned to him by Dr. Kamikazi.
- Donnie Turnbull - Tommy's older Brother and archenemy, he is a constant bully to every child in the neighborhood, including Gus and Tommy himself. He frequently calls people names, threatens them etc., and yet despite this, Tommy still treats him as family. Some people exploit his attitude for their benefit, usually Dr. Kamikazi. His bullying backfires in the third season after Robotboy tortures him when he decides to play hooky from school in order to humiliate and possibly destroy Robotboy.
- Kurt - A sadistic bully at Tommy's school, he has blonde hair and wears a hat. He constantly bullies Gus and Tommy, as well as any other kid he finds. His dad is a spy who has used Kurt to try and capture Robotboy. Tommy sometimes has outsmarted Kurt and his cronies by outwitting them, usually with Robotboy.
- Stu - One of Kurt's friends/henchmen. He is the largest and least intelligent of the group. He wears a blue hooded top with a red cap.
- Mookie - One of Kurt's friends/henchmen. He is the smallest bully. In the absence of Kurt, he is the leader of the group. He wears a dark red beanie which happens to have a bowling ball hidden inside.
- Bambi - Bambi is the popular girl at Tommy's school and Kurt's girlfriend. She is blond with pigtails and can be seen wearing a cheerleader's outfit. Tommy blushes and is unable to talk when he sees her, much to Lola's dismay. She was once replaced by a robot by Dr. Kamikazi in an effort to defeat Robotboy. She seems to be very spiteful and mean towards Tommy, yet he still likes her. She is spoiled by her single mother, and is also rather 'air-headed.' She is voiced by Laurence Bouvard.
- Björn Bjornson - An evil boy who is about Tommy's age and claims to be a greater genius than professor Moshimo. He has his own prototype robot named Brother Björn orBjorn-Bot. Both were first introduced in the episode of the same name - "Brother Björn". Their mission is to destroy Moshimo's creation - Robotboy, so Bjorn-Bot would remain the only fighting robot in the world.
- Bjonbot - Bjornbot is a robotic lookalike of his creator Bjorn Bjornson. He has weapon capabilities and strength equal to that of Robotboy, and can Superactivate too. It seems that Bjorn himself has created several Bjornbots. Bjornbot has his own form of Superactivation. Unlike Robotboy, he lacks emotion and has a rather flat vocabulary (which consist mostly of the word "Ja!") and Robotboy thinks of strategies in battles and Robotboy is able to overcome Bjornbot. In the episode Party Out of Bounds he dressed like a girl and Bjorn called him Sister Bjorn. In more recent episodes, he seems to have become more hyperactive, being very jumpy and excited. He is voiced by Lewis MacLeod.
- Protoboy - Robotboy's evil brother, who was created by Moshimo in his earlier years, but later stolen by young Kamikazi and Constantine and turned too evil to control. He was then forcibly deactivated by Constantine and stored in a large storage room inside of Kamikazi's quarters in Kaziland until he was saved by Robotboy. His first appearance was in episode "Brother", but he returned in episode "The Old Switcharobot" and was later permanently destroyed by Robotboy in "The Return of Robotgirl" - the last episode of the series. He also has the ability to superactivate just like Robotboy, but the cannon built into his left arm is static and cannot be switched with an arm or another weapon. Robotboy was seen superactivated in Protoboy's body in "The old swticharobot" but his cannon was damaged because Protoboy (in Robotboy's body) had removed some of its parts prior to the body switch.
- Klaus von Affenkugel - Affenkugel lives a life that is blessed and cursed. He is obscenly wealthy to the degree he has his own form of currency. He lives in a large castle and could afford to buy the very best of everything. Yet desires the one thing his money cannot truly buy, a strong body. Affenkugel now dedicates his money to capturing Robotboy so he can use his robot parts as power armour for himself.
- Ludwig - After being bullied by both kids and squirrels, Klaus went near a zoo, where a missing ape, Ludwig, decided to protect him and give him company. He is a large ape who serves as the assistant to Klaus at all times.
- Felonious Hexx - Is a magician. He wowed the crowd with incredible feats of secretly real magic. However Gus, being his usual self, was not impressed with the show no matter how amazed the rest of the audience was, and continually jeered, insulted and mocked the otherwise brilliant magician. This lead Hexx to become angry and lash-out with his magic.Which after a scuffle with Robotboy got him fired from his job and kept him from black balled from performing ever again. From then on, Hexx became Gus's arch-rival.
- Dr Freeman Coldpepper - Tommy's Principal and nemesis. She is a liar and is obsessed with toys due to never being allowed to play with them as a child, instead having to make her own dolls out of tin foil and chicken bones. She abuses her position as principal to confiscate toys and play with them herself. She later returns for revenge to ruin Robotboy's cliffhanger and play with him forever.
- Mr. Yashinski Coldpeer's henchman.