Duo was a crew member aboard an unnamed hovercraft, along with Cis and Kaiser. However, he is actually a Program that serves as Cis' foil.
Cis and Duo went inside a training simulation that resembles Feudal Japan.
Behind the Scenes
He, along with the rest of the crewmembers appear in the Animatrix short story known as Program. He was voiced by Phil LaMarr.