Duo is a powerful extra-terrestrial robot that appears in Mega Man 8. His exact origins are unknown, but he claims to be an interstellar peacekeeper that exists solely to rid the universe of Evil Energy, an unknown form of malignant energy that can infect the minds of intelligent beings, corrupting them and multiplying to spread to others. Duo is capable of tracking Evil Energy wherever it is and uses his bulky left arm to grasp sources of the energy and nullify it.
In Mega Man 8, Duo falls to Earth following a battle against an evil alien robot that harnesses Evil Energy. The evil robot is destroyed but Duo is badly damaged and falls into the planet's atmosphere, crashing upon a small skull-shaped island. Both Dr. Wily and Dr. Light detect what they believe is a meteor which gives off a tremendous energy signature. Fearing Wily would abuse such power, Dr. Light sends Mega Man to find the meteor and bring it back to Light Labs. Both Mega Man and Wily find the supposed meteor and Wily escapes with the Evil Energy, but Duo still lies within the impact crater and Mega Man takes him home. While Mega Man heads out to thwart Dr. Wily's robots, Dr. Light examines Duo back at the lab and tries to repair him. Duo later awakens and notices several Evil Energy resources that Mega Man retrieved from Wily's robots. Believing his alien foe still lives, Duo destroys the Evil Energy capsules before breaking out of Light Labs. Mega Man and Rush go after him and their chase also leads them to an underground volcano where Dr. Wily has established a new base.
Duo reveals his good nature when he rescues Mega Man from one of Dr. Wily's giant robots. He explains to the Blue Bomber that he came to Earth in pursuit of the Evil Energy that Wily had acquired and that the evil within Wily's mind will cause the energy to multiply and consume the entire planet within days. Both robots agree to work together from then on; while Mega Man hunts down the barrier generators for Wily Tower, Duo will eliminate the Evil Energy that has been spreading across the world. Eventually, Duo finishes his cleanup of the Evil Energy and invades Wily Tower after Mega Man has already breached the defenses. When he arrives, he finds that Dr. Wily has been defeated and tears apart the base. Unfortunately, Mega Man was infected by the Evil Energy during the final battle and was close to death. Duo used his power to remove the Evil Energy and saw into Mega Man's mind as he healed him, witnessing all of the heroic deeds he had performed. After removing the evil from Mega Man's mind, Duo left Earth to continue his quest, but not before telling Proto Man to give Mega Man a message of thanks from him.
- Superior Strength - Duo possesses incredible physical strength, even greater than any of the Robot Masters built by Dr. Light or Dr. Wily.
- Flight - Duo's jets allow him to fly at extreme velocities through air and space.
- Energy Attacks - Duo can surround himself with energy to perform a crushing charge attack. He also uses his huge left hand to fire fist-shaped projectiles or produce devastating shockwaves.
- Evil Energy Purge - Being made for the purpose of destroying evil energy, Duo is capable of removing said energy from subjects and nullifying it completely.
- Duo has also appeared in Mega Man Battle & Chase and Mega Man 2: Power Fighters as a playable character.
- In the Megaman Gigamix manga, Duo's origins are much less alien. He was created by Dr. Cossack based on data retrieved from an alien robot that was responsible for imprisoning the Stardroids and was created to defeat them when they threatened to destroy Earth.
- Duo also made a brief appearance in Archie Comics' first Sonic/Mega Man crossover event, Worlds Collide.