Zero Nightmare

The Zero Nightmare (ゼロ ナイトメア) is an insane off-colored copy of Zero created by Gate, through use of Zero's DNA. When he is defeated by X, Zero will become a playable character. He is weak to the Z-Saber, Magma Blade (since it utilizes the Z-Saber, but only the saber part can hit him), and Rekkoha.

Characteristics
The Zero Nightmare is a discolored palette swap of Zero's sprites from Mega Man X5.

Zero Nightmare uses imperfect variations of attacks used by 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 Genmu Zero Kai, a "revised" version of an attack used only by Awakened Zero. However, this variation is not an instant kill, and is only one wave, rather than two. It is also subject to the same odd randomness in speed. He can also teleport, 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. Note that this only works if X is standing when swinging the Z-Saber, and if he is jumping or on a wall (either sliding down or hanging on using the Shadow Armor), Zero Nightmare will just be damaged normally.

Zero Nightmare seems to be a botched project, physically and mentally unstable. 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... KILL... KILL!" among other incoherent phrases in a mindless 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. 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. 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.

History
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, used the fraudulent claim of Zero going on a rampage for Gate's advantage—fully knowing that Gate was the true perpetrator. 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, for unknown reasons. 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 as nothing more than a faux to give credibility to the claim. The real Zero decided to keep out of sight as the Zero Nightmare tarnished his name, and only revealed himself once X had disposed of the clone.

Power and Abilities
Zero Nightmare has the same exact arsenal as Zero from Mega Man X5, using both his Z-Saber and Z-Buster to attack, only with a limited movepool and subsequent speed.

Note: With the exception of Shin Messenkou (真滅閃光) and Genmurei Kai (幻夢零改), none of these attack names are official.
 * Teleportation: Zero Nightmare can warp around the room in a dashing stance.
 * Z-Saber Slash: Zero Nightmare hold his Z-Saber up and after approximately half a second, he swings it.
 * Z-Buster Shots: Zero Nightmare shoots two shots that go straight ahead.
 * Thousand Shots: Zero Nightmare shoots a large barrage of charge shots from his Z-Buster.
 * Denharei: Zero Nightmare launches an energy wave that is semi-homing from his saber after using Z-Buster, similar to Zero's boss patterns in X2.
 * Shin Messenkou: Zero Nightmare punches the ground, making several energy projectiles fly up from the floor.
 * Genmurei Kai: Zero Nightmare attacks with a weaker version of Awakening Zero's final attack, firing off single giant energy waves at varying speeds.

Music theme
His boss theme is X vs. Zero, which is the same theme used in the boss fight against X (Zero's storyline) and/or Zero/Awakened Zero (X's storyline, depending on the player's actions beforehand) in Mega Man X5.

Trivia

 * If a battle between the real Zero and the Zero Nightmare is triggered through the use of a cheating device, the Zero Nightmare will use the quotes of High Max, presumably because the dialogue was never programmed for an exchange between Zero and the Zero Nightmare, resulting in the game loading the next available dialogue in its programming: that of High Max. The only place this does not happen is in Commander Yammark's stage, where it will reveal an unused dialogue rather than using High Max's. The dialogue is a variant to X's encounter with the Zero Nightmare. The reason for Zero's name changing to X in his second line is likely due to an oversight, hinting that it would be possible for Zero to encounter the Zero Nightmare at one point:
 * Zero: Who are you!
 * Zero Nightmare: It's me. Don't you recognize me?
 * Zero: (Name switches to X) ...You are pitiful. What do you want?
 * Zero Nightmare: I've been looking for my hunters... To wipe out! Time to die now!
 * One very notable line that Zero Nightmare says upon encountering him is "WHERE IS ZERO? WHERE IS HE HIDING? A BLUE ROBOT. ENEMY OF ZERO...KILL YOU!" It is interesting in that while searching for Zero, he identifies X as Zero's enemy. It is possible that along with some memories from Zero, he may possibly have inherited some of Zero's memory of his mission to destroy X from the Maverick Virus. This is also supported by his imperfect usage of Genmu Zero, an attack used only by Awakened Zero in X5. The conflict of memories could be the cause for his mental instability.
 * On the other hand, with the aforementioned quote, Zero Nightmare's dialogue may not in fact be referencing X at all, as the stated quote could be a reference to yet another one of Zero's original memories: his original purpose of destroying Mega Man, as programmed by Dr. Wily. Considering the insane nature Zero Nightmare exhibits, it is highly plausible to suggest that Zero Nightmare confused X with Mega Man. This is also supported by Zero's terminology, describing X as "A BLUE ROBOT." To call X a robot is rather peculiar because, by the time of X6, this term has been replaced with "Reploid" long ago, giving merit to the believe that Zero Nightmare did in fact reference Mega Man by mistake.
 * In the Japanese version of the game, Zero Nightmare refers to himself as "washi" (ワシ) when saying "WHERE IS MY ZERO!?", which is the Japanese pronoun typically used by people of old age. This implies that Zero Nightmare is somehow confusing Dr. Wily's memories with Zero's.
 * Due to a programming oversight, Zero Nightmare is impossible to fight at level 4, since bosses will only be at level 4 when the PA rank is achieved, which requires 5000 Nightmare Souls, and one way to unlock the endgame is to collect 3000 Nightmare Souls, meaning that fighting Zero Nightmare and High Max will no longer be possible. The only way to achieve this is through cheating devices such as GameShark. (The NTSC version of the code to fight Zero Nightmare and other bosses at level 4 is 300CCF5F 0003﻿)
 * If Zero Nightmare is defeated when it is at level 4, players are rewarded with the Black Zero enhancement without the use of codes. It is worth noting that there is no special text or notification if the player achieves the enhancement through this method.
 * Zero Nightmare makes a cameo appearance in Marvel vs. Capcom: Infinite as Zero's fourth color palette.