Evil Robot

Evil Robot (悪のロボット) is an alien robot that Duo was fighting with in the beginning of Mega Man 8. The robot is an untouched character in terms of backstory, but important nonetheless. He was destroyed by Duo in a battle which resulted in the two robots crashing to Earth. Duo managed to damage him to the point where his body went completely inert, but the evil robot did enough damage to Duo where neither one could slow down to avoid burning up upon reaching Earth. Though the evil robot was destroyed, the Evil Energy it harnessed lived on, which is what Duo spent the remainder of the game trying to destroy. His appearance is almost identical to Duo's original form, albeit mirrored (opposite giant hands to Duo) and with a more "evil" looking armor and color-scheme. His head is also different, with slots under his eyes rather than Duo's T-Visor.

Mega Man (Archie Comics)
The evil robot has a brief appearance in Issue 42, where Shadow Man, when reflecting on his past, remembers his being confronted by a similar robot to Duo belonging to a group called the Star Marshals. The evil robot ends up saving Shadow Man by stabbing the other in the back with his arm. Going by the name Trio, he defects from the Star Marshals after seeing the power promised by Ra Moon. He allows Shadow Man to escape and take Ra Moon to Earth while he makes up an excuse to his superiors.

Other appearances
The evil robot has a short appearance in Mega Man Gigamix and the Rockman 8 manga.

Trivia

 * Some fans refer to the evil robot as "Oud" due to his appearance being a mirror of Duo, and some refer to him as "Duo", believing it and Duo to be two halves of a robot.