|First Appearence:||Megaman X|
Sigma used to be the leader of the Maverick Hunters, and was believed to be immune to the influences of the Maverick Virus. After a fight with Zero when he was a maverick, Zero transmitted the virus to Sigma after Sigma shatters Zero's forehead crystal. He eventually turned into a Maverick himself, and became the leader of the mavericks. He then launched his attack against the humans, but then his plans were thwarted by a reploid named MegaMan X, and was eventually defeated by X.
Six months later, during the Maverick project "Unification" as the Mavericks try to turn Zero maverick, they had a back up plan, and that is to revive Sigma. As the Unification never transpired, the revival of Sigma was a success, but again X defeats Sigma.
Durign his second revival, he transformed into a virus to try to corrupt reploids and make them mavericks, he eventually would corrupt Dr. Doppler and his Neuro Computer and make his army attack the world. The corrupted Dr. Doppler had also made the ultimate battle body for Sigma so he can take on a physical form again. But X and Zero stopped Sigma's plans once again, even with the powerful battle body.
But Sigma was revived again after a few months, and found about a new reploid army known as Repliforce. He even wanted to see the leader General, to get the Repliforce fight Maverick Hunter, but General refuses and dismisses Sigma. Shortly afterwards, he secretly led to the attack of the Sky Lagoon, and made sure that Repliforce was held responsible for it. But, he would eventually come out of hiding after the Maverick Hunters defeated Repliforce, and tries to defeat X and Zero, but the duo defeat the maverick again.
Then decades passed, and Sigma was revived again. This time he had his eyes set on the human orbital colonization program, mainly the biggest space colony: Eurasia. He hired a reploid bounty hunter named, Dynamo to make it fall and aim it for Earth. He also learns the secret of Zero's creation, as he intends to spread his virus all over the planet and reveal Zero's true form. The Maverick Hunter destroyed the colony, only to have its remains fall towards the earth. He and X fight Sigma again, and Zero, with his last breath, defeated the maverick.
3 weeks after the Eurasia incident, Sigma was thought to have been defeated for good, only to have been yet again revived, this time from an eccentric reploid scientist named Gate. However, he was not complete when X and Zero faced him, making him easy to defeat.
After he went for his most recent revival, he attacked the Earth, and left it in ruins, causing the rest of the global population to evacuate to the Moon via the Orbital Elevator. However, the trio (X, Zero and Axl) defeated the maverick yet again. After his eighth defeat, rumor spread that X destroyed the true source of sigma, which means that he may never return again.
 Other Appearances
Sigma can also be used as a reference to referring to he all powerful Sigma Bosses in Mega Man Starforce 3: Black Ace or Mega Man Starforce 3: Red Joker. The Sigma bosses that appear in both games would consist of Moon Destroyer, Sirius, and finally Apollo Flame. These are known as the hardest bosses in the game and to make them appear in game you have to tap the badges on the title screen in a certain order after obtaining them. This order consists of: S Comp Star, G Comp Star, S Comp Star, M Comp Star, SS Star, SS Star, and Black Ace or Red Joker star.
This code will trigger for the bosses to appear randomly without a warning sign in certain places. These are known to be the hardest bosses in the game and if you beat them all then players will fear the one who has the Sigma badge on Wifi. Also there is a Crimson Dragon in a Sigma form but you get nothing in return for beating him other than a nice 6000 zenni's which you won't probably have a use for anymore at that point in the game.
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