From The Mega Man Universe
The Zero Nightmare is a discolored pallet swap of Zero's X5 sprites.
Zero Nightmare uses imperfect variations of attacks used by Hunter Zero in X5, but with random shot speeds. Most notable is a delay when he swings his saber, where he holds it above his head before coming down, and his use of Genmurei, an attack used only by Awakening Zero. However, This variation is not instant kill, and is only one wave, rather than two. It is subject to the same odd randomness in speed. He has the ability to teleport in a similar fashion to Nightmares, and does so frequently in battle.
Curiously, and ironically, Zero Nightmare is actually weak to Zero's Z-Saber, causing him to flinch and cry out in pain when hit with it, after which he immediately teleports away from X to another spot.
Zero Nightmare seems to be mentally unstable, almost a botched project. At times he would ask X where Zero was, seeking him. Other times he would claim to be Zero himself, and yet other times, he would yell "BLUE LIGHT MUST KILL" among other incoherent phrases in a senseless rage. No matter what, though, X would deny his claims or snap at the "cheesy fake," and Zero Nightmare would instantly go into a rage bent on killing X, or claim X has gone Maverick and that he will retire him. While some memories and attack patterns seem to have remained from different periods of Zero's life, the clone was not complete in any respect, and was therefore easy for X to see through as a forgery of his longtime friend.
Zero Nightmare was created by Gate and used by Isoc and High Max's investigation teams to justify their existence. The two claimed to want to get to the bottom of the Nightmares, but in reality knew the origins of it as Gate. Created from Zero's DNA, Zero Nightmare seemed to retain some of Zero's memories, but at the same time sought out the real Zero. The investigators were to have the public believe that Zero had fallen under the influence of the dreaded Nightmare, but Zero Nightmare had simply been created that way. The real Zero kept out of sight as the Zero Nightmare tarnished his name, and only revealed himself once X had disposed of the clone.
 Battle Data
 Notes & Trivia