An robot made on an alien planet, Duo is designed to eradicate "Evil Energy", chasing the adult stage of evil energy in its final form. After crashing to Earth, he is repaired by Dr. Light, and soon becomes an ally of Mega Man. After finally defeating the evil energy on Earth, he thanks Mega Man and leaves Earth to resume his search.
Hayato Kaji, the artist responsible for Duo's design, commented that Duo "started off as a robot created by Cossack, so there’s still a lot of Russian flavor left in him. The shape of his head was supposed to look like a classic Russian hat, and the circles on his chest were meant to look like coat buttons…".
The bulk of Duo's offensive abilities are attributed to his powerful left arm, with a fist nearly as large as his own torso, and his large spiked body. He attacks primarily by punching, though he is capable of using projectile weapons, the most powerful of which is a blast in the shape of his own hand.
Duo also appears to have the same weapon copying abilities displayed by Mega Man and Proto Man, and like them his body changes color in accordance with the weapon he has equipped. Also, having been created for the purpose of destroying evil energy, he seems to have the unique ability to purge this energy from an infected victim's body, as demonstrated when saving Mega Man.
- In the cancelled Rockman Online, a government-engineered copy of Duo made to combat a Maverick epidemic joined forces with the Mega Man X team.