Omega

Omega, dubbed "the devil Reploid" or "the ultimate Reploid" is a massive Reploid created during the ending period of the Elf War. He is Dr. Weil's greatest creation and was sent into battle as a last resort action during the war's closing period. He was created to store and manipulate the Dark Elf in order to mass-control Reploids by remote. He plays a major role in the story of Mega Man Zero 3 and has a cameo appearance in Mega Man ZX.

Personality
Omega does not hold very much of a personality, beside that of being a blood-thirstly tool of Dr. Weil's ambitions and absolutely obedient to his commands. After returning to Earth after spending a century of exile in space, he developed a grudge against Zero, because he was still trying to best him though he no longer possessed his original body. Also, Omega's ability to speak was reduced to minimum (at least while in exterior armor). Other than that, he doubtlessly believes in Weil's insane purpose for him, being his messiah of the new world order and "God of Destruction".

The Elf War
After the Maverick War had been ended by the powers of the Mother Elf by reverting the viral infection of Reploids caused by the Sigma Virus, Dr. Weil, a weapon researcher and DNA revival specialist who had worked at the same laboratory as an unknown ancestor of Miss Ciel (who had researched Zero's body) in order to find a way to end the war, felt unsatisfied with the result of the solution. Thinking that it wasn't enough to simply cure the Reploids from the virus while they posed still a threat to society due to their free will, he decided to create his own, radical solution: To exterminate every Reploid that did not decide to submit to the humans and regard them as their creators and masters.

To achieve this goal, he had to find someone capable of fullfill his desire, a "messiah" that would destroy every Maverick and teach the Reploids to know their place. For that position, Weil had chosen Zero, who's mindless body was still in his laboratory. He stole the body as well as the Mother Elf. Weil corrupted the Mother Elf, turning her into the Dark Elf to control all the world's Reploids at the same time in order to have them destroy each other. However, even in this corrupted state, the Dark Elf was not powerful enough to control all Reploids at the same time and the Babyelves, small copies of the Dark Elf, were created.

Yet, as Weil desired to use the Dark Elf alone as well as his new messiah, he began constructing a massive exterior armor around the mindless body. The armor boosted the body's attack strength to its limits (although at the cost of manuverabilty and evasiveness) as well as granting it the ability of using the Dark Elf to control all Reploids at the same time, making the Babyelves useless. The body received new weapons, suitable for its size and weight and was given a new conciousness loyal to Weil only: Omega. When the war had been going on for almost four years and had taken the lives of 60% of all humans and 90% of all Reploids, the real Zero, whose mind had remained in the goverment's laboratory, reappeared on the battlefield, given a dublicate of his old body. It was him who managed to intrude into Weil's lab and steal the Dark Elf, which he commanded to use have all Reploids close enough to be controlled to turn against Weil. As a final act of trying to win the war against the Maverick Hunters and the goverment, Weil sent out his greatest creation, though he hadn't been able to absorb the Dark Elf. He was defeated by X and Zero (who, at that point, had realized it Omega was in possession of his original body) by using Final Strike.

After the war and the punishment of Weil by the survivors, the court of the newly established Neo Arcadia, the Eight Gentle Judges voted against destroying Omega as he had been manipulated by Weil alone. Instead, he was sealed away in a spaceship and sent into space, doomed to spent all eternity in exile. This verdict however, was manipulated by Weil as the Eight Judges actually were his creations (later called "Weil's Numbers") and he ordered them to not destroy Omega in case he was captured).

Neo Arcadia era
Under Construction

Mega Man ZX
Omega appears as an optimal boss character in Area N of the game. Due to the massive explosion his destruction had caused, a rift between reality and Cyberspace was caused, similiar to what happend when Omega returned to Earth after his exile. The Omega appearing in Area N is not a rebuilt version of the Reploid but actually a corrupted memory file that remained of Omega when he was destroyed. This is believed to be the case because of the environment around him constantly changing between the normal background of Area N and that of the final battle of Zero and Omega. After he is destroyed, he will leave an item called "Mystery Stone". After clearing the game once, the stone can be taken to Fleuve, who'll analyse the stone and transform it into Biometal O which lets Vent/Aile become Omega, including all powers and attacks he possessed.

Trivia

 * Before Zero battles with Omega Zero, the latter shouts a Japanese line, "Ware wa meshia nari!" This translates to "I am the messiah!" Omega Zero uses the same line before the player finds and fight him in Mega Man ZX. This fortifies the fact that many of the characters in the Zero series referred to Omega as "The God of Destruction."
 * Omega's armor in the beginning of Zero series emits a glow from the black parts that make up his visor, face, chest, and abdominal region upon defeat, which then seem to heal the armor, and bring the claws upward from the ground back to a levitating position where real arms would be. This could mean that his armor was made with regenerative capabilities. This makes sense in that if the armor were broken, it would have released Omega in his most powerful state (when he was in control of Zero's original body). The regeneration might be to keep the armor active and the armor itself seals and limits Omega's power extensively.
 * It is currently unknown why Zero's original body appears visually different in "Mega Man Zero 3" than it did throughout the X series titles. One would logically assume that his "original" body would have the same appearence that he had in the X series, while his "copy body" retained its look from the Zero series. Some fans speculate that the differences merely reflect the change in art style and should not be considered a factor in the overall story, while others believe that his original body's appearence could have been modified during the time period when it was being researched in order to create the Mother Elf. This could also theoretically account for the look of the copy-body as it would have been modeled off of the most recent visual appearence of the original body. Currently, however, no official canonical explaination for this mystery has been offered by Capcom. The only definitive statements, which were made in the Offial Complete Works sourcebook, suggest that Zero's visual appearence was changed by the Zero series artists to make him appear "more human".
 * It is unknown if Omega's Biometal form in Mega Man ZX is part of the canon story or simply a gameplay extra.