Protoboy is a reoccurring villain in Robotboy. He was created by Professor Moshimo in his earlier years. He has many of the same qualities as Robotboy, especially advanced fighting skills, but he lacked Robotboy's humanity. He was stolen by a young Dr. Kamikazi and Constantine. Kamikazi reprogrammed Protoboy and turned him evil, however, Kamikazi made Protoboy too evil and couldn't control him, causing Protoboy to go on a rampage and destroy Kaziland. Protoboy was disconnected by Constantine and placed in a reject closet.
It wouldn’t be until years later that Robotboy, after learning about him from Moshimo, would reactivate him in an effort to turn him good, his efforts proved futile and he eventually defeated Protoboy with a limpet mine, which caused an explosion. However he survived and later returned to get his revenge on his brother and creator.
In the episode "Brother", after hearing about Protoboy, Robotboy, being sad that he had no family of his own, traveled to Kamikazi's lair and reactivated Protoboy, who then began destroying the city. When Robotboy superactivated, Protoboy did as well. Robotboy was nearly defeated until he used the new weapon Moshimo gave him, which is essentially a limpet mine. Which caused Protoboy to explode.
He wasn't seen again (chronologically) until "The Revenge of Protoboy" when he tried to killed Robotboy, Tommy and Gus with an asteroid, after Robotboy destroyed it, he nearly killed him by ripping him apart and would have if he didn’t get a phone call from his "dear". He later tried to sink a cruise ship that Tommy's parents were on, and when Robotboy refused to fight him and tried to get him away from the ship, he captured Tommy implicitly strangled him with his shirt and made Robotboy choose between saving him or the boat to get him to fight. Robotboy manages to save both, then he and Protoboy briefly fight. Robotboy plays possum to make Protoboy think that he destroyed him, he then ripped Protoboy's motherboard out. Protoboy then deactivated and fell into the sea. It is in this episode where it is revealed that he may have a wife or romantic partner.
He returned again in "The Old Switcharobot" when he switched bodies with Robotboy (only ending up with Robotboy's circuits by accident.) This is also the episode in which Robotboy receives his new body. As Brainy Yak destroys Protoboy in Robotboy's old body.
But he returned once more and for the last time in "The Return of Robotgirl" where he tried to destroy Moshimo and Robotboy. He kicked out Robotgirl's batteries and almost defeated all three of them, but was foiled when Robotboy and Moshimo broke free and destroyed the bridge that Protoboy was standing on. Protoboy seemed to have melted in the molten lava he used to try to destroy Moshimo and his brother, but is not shown falling in. Leaving his fate unknown. Protoboy could have been a great brother if he wasn't turned evil. He is stronger than Robotboy but he doesn't get any updates and his smarts is almost as great as Professor Moshimo. Protoboy can be captured while he's turned off and re-programed.
Protoboy is shown to be a cold, emotionless and sadistic killing machine. He takes pleasure in destroying whatever he can and considers it "fun". He is shown to enjoy fighting, as he once said fighting Robotboy would be exciting for him. He almost never shows any emotion other than anger and sadistic joy.
Protoboy can also be calculating and deceitful, as in "The Old Switcharobot" where he swapped bodies with Robotboy in an attempt to be able to kill Moshimo and "The Return of Robotgirl" where he lured Robotboy and Robotgirl to him.
He is the most popular villain in the series.
He and Bjornbot are the only reoccurring villains to unambiguously die, although they both get revived later on.
Writer Robert Mittenthal stated that they may have had an idea to make Protoboy good again if the series continued, meaning that he could have redeemed himself if the show carried on.