Protoboy is a robot who created by Dr. 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's circuits and turned him evil, however, Kamikaze made Protoboy too evil and couldn't control him. Protoboy was disconnected by Constantine and placed in a reject closet.
In the episode "Brother", after hearing about Protoboy, Robotboy, being sad that he had no family of his own, traveled to Kamikaze's lair and re-activated Protoboy, who then began destroying the city. When Robotboy used Super-Activation, Protoboy did as well. Robotboy was nearly defeated until he used the new weapon Dr. Moshimo gave him, which is essentially a limpet mine.
As a result, Protoboy exploded and wasn't seen again until "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 unbreakable armour.
He then returned in "The Revenge of Protoboy" when he nearly disabled Robotboy, tried to sink a cruise ship that Tommy's parents were on. When Robotboy played possum after he fell into the sea, he ripped Protoboy's circuits out. Protoboy then deactivated and fell into the sea. It is in this episode when it is revealed that he may have a wife.
But he returned once more and for the last time in "The Return of Robotgirl" where he tried to destroy Moshimo and Robotboy. He melted Robotgirl's batteries and almost defeated all three of them, but was foiled when Robotboy broke free and destroyed the bridge that Protoboy was standing on. Protoboy seemed to have melted in molten lava he used to try to destroy Moshimo and his brother, but is not shown falling in. 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 Dr. Moshimo. Protoboy can be captured while he's turned off and re-programed.