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In reality, Isoc was using this fraudulent claim of Zero's rampage as an excuse to get Gate's servants positioned around the world. He was used by Isoc to justify the existences of High Max and the Nightmare Investigators, describing him as a "ghost of Zero" that was on a rampage thanks to the Nightmare.
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Zero Nightmare was created by Gate using the Nightmare and Zero's DNA. While some memories and attack patterns seem to have remained from different periods of Zero's life, the clone was incomplete in any respect, and was therefore easy for X to see through as a forgery of his longtime friend. These conflicting and random exchanges of dialogue suggest that the Zero Nightmare has some of Zero's memories from throughout Zero's life (including the times with his original programming), but without context, he simply acts on his base instincts, which compel him to kill X and any others who stand in his way, eerily similar to the Maverick state Zero was first found in. 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. KILL!" among other incoherent phrases in a mindless rage. Other times he would claim to be Zero himself, and yet other times, he would yell "BLUE LIGHT. At times, he would ask X where Zero was, seeking him. Zero Nightmare appears to be a failed project, both physically and mentally unstable. In Mega Man X DiVE, Zero Nightmare is also shown with one sparking purple eye and glitch-like effects surrounding his body. His entire body is purple in color, while the gems on his body are green and his Z-Saber is a grey-ish purple. The Zero Nightmare is a discolored palette swap of Zero's sprites from Mega Man X5.
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