Sigma is the primary antagonist of the Mega Man X series by Capcom. He was the leader of the Maverick Hunters, before succumbing to a flaw in his programming. He leads all haywire Reploids on waging war against humanity.
Sigma appears as a bulky, bald humanoid in most of his forms; his most pronounced feature is a crystal on his forehead, and two parallel scars running through his eyes. He often produces a second form after his initial defeat in a game, which frequently has the head (or the crystal on it) as a weak point.
Although Sigma has taken many drastically diverse forms throughout the X series, his humanoid design remains fairly consistent, resembling a giant bald human with a cleft chin.
Sigma was originally an honorable and selfless leader of the Maverick Hunters, but after the corruption of Maverick virus inside his body since his last fight against Zero (who at the time was a Maverick), he became a power-hungry tyrant, although even then, he also genuinely believed that he was doing what was in the best interests of the Reploids. He typically looks down on others and does not hesitate to kill the weak, laughing maniacally at their expense. His previous "protection through aggression" philosophy has mutated following his infection into a huge Darwinism complex, believing that anything that is too weak to progress should be annihilated, a la natural selection. Following his defeat, his further descent into madness transcended death, trying to hold onto any grip in the living world, in any form possible, consumed by the lingering thought of killing X. Amidst his madness, Sigma is a calculating and intelligent individual, and on more than one occasion, turned allies of X and Zero against them, and instigated many of the most destructive events of the Maverick Wars, including the Earth Crisis. Sigma is obsessed with Reploid evolution and superiority, and believes humanity has run its course. Despite his hatred for X and Zero, Sigma once believed they were the future of all Reploids, and would lead them all into a new age. In addition, he also credited X's dedication to justice, as well as his being infected with Zero's virus, as being his primary motivation in his going Maverick, including what ultimately turned out to be his final battle on the Moon.
As the former leader of the Maverick Hunters, Sigma is a capable military leader, described as possessing overwhelming combat power and strategic intelligence. When he turned Maverick, Sigma remains a charismatic leader, and is revived on many occasions by Reploid and Maverick admirers alike. Due to the virus in his circuits, he can survive seemingly anything and always return for more. He is a very powerful Reploid, and is fond of incorporating himself into battle suits which are vastly larger and more powerful than his "regular" form. Sigma is also a skilled manipulator, managing to turn various Reploids, Maverick Hunters, and Mavericks to his side. In fact, there were few instances in the entire series where Sigma wasn't pulling the strings. Sigma himself is one of the strongest Reploids ever created, due to his original role as leader of the Maverick Hunters. His personal energy has been stated to be too powerful to even get an accurate reading. In addition, Sigma's combat skills and analytical abilities were extensive enough that he was able to wipe out several reploids in an efficient manner, as Zero noted when investigating the crime scenes on the days leading to Sigma's revolt.
Sigma is capable of moving at great speed, often able to move faster than the hero can follow, vanishing from sight. Many of his bodies are also capable of teleportation. Sigma seems to favor saber and beam sword weaponry, often integrating powerful sword combos with his blinding speed or teleportation. On several occasions, he projects energy barriers around himself to block incoming attacks. In nearly every appearance, Sigma is capable of discharging enormous destructive blasts or waves of power capable of covering entire battle areas. Sigma's physical strength is considerable. In his first body, during a conflict with the original Maverick Zero, he was able to effortlessly break through Zero's defenses with a round kick, as well as easily toss Zero through the ceiling with a single hand. He has also demonstrated the ability to scale a skyscraper with a single leap.
Sometime prior to Sigma's first defeat at the hands of X, his reploid soul became intermingled with the dangerous Maverick Virus and formed the Sigma Virus. This allowed Sigma's spirit to remain after his body was destroyed, granting him immortality and the ability to return after the destruction of each subsequent body. The Sigma Virus possesses the ability to travel through any material or energy and can manipulate both reploid and organic hosts, even on a planetary scale. Its intangibility allows it to ignore physical assaults and energy attacks, except for those tuned with a Virus Buster to target the Sigma Virus's unique matrix. No solution for the Virus remained ultimately effective during 21XX-22XX, as Sigma demonstrated the ability to adapt to any countermeasure. As the cause of death and suffering for countless innocent lives, Sigma is one of the most dangerous villains in the Classic Mega Man Universe.
Sigma first appeared to act hostile towards Ultron, whom the former thought about the latter being a simple robot as the rest of Reploids. However, Sigma realized that Ultron's programming code is the same as his, such as their sharing mindsets on putting all biological lifeforms into despair. Ultron considers Sigma's design is a simple mockery of the real human flesh, in which Sigma admitted. As Thanos is currently watching them, Sigma and Ultron devices a plan to betray Thanos, then uses both Space and Reality Stones to converged every timeline in two worlds. Once Thanos is brought to Sigma's lab by Ultron and Sigma, the two latters initiates the plan, with Ultron stabs Sigma's body to upload his new Maverick partner's consciousness into his via Reality Stone, and thus... Ultron Sigma was born.
Using his energy sword, Sigma is able to control the battle by opening up dimensional rifts that linger around the screen, hindering his opponent's movement options. Armed with a command grab and a counter teleport, he has options to deal with overly aggressive teams that may try to take advantage of his slower but imposing move set.