Ultron Sigma is the merged form of Ultron and Sigma that appears as the main antagonist in Marvel vs. Capcom: Infinite. He is voiced by Jim Meskimen and Chris Tergliafera, the voice actors for Ultron and Sigma, respectively. Their "voice" are that of both Ultron and Sigma's voices overlapping each other, owing to them being fused from the two characters. As with previous Megaman X titles, Ultron Sigma also evolves into a final form in the final battle, known as Ultron Omega.
Ultron Sigma takes design aesthetics from their namesakes; featuring Ultron's smooth and futuristic textures, yet possessing Sigma's general bulky body shape spike-studded limbs and red cape, their overall color scheme being black and gold. Their head is identical to Ultron's but exhibits a purple-ish glow as well as lining around their eyes reminiscient to Sigma's trademark eye scars, with Sigma's facade spread across his torso, featuring blue eyes. On both of their "faces" are the Reality and Space stones, respectively.
After Ultron and Thanos uses the Reality and Space Stones to enter Capcom Universe's Abel City, Ultron hacked Abel City's barrier system and meet Sigma alone where the two villainous robot formed an alliance for same method besides betraying Thanos, elimination of all organic life. At Sigma's Lab where Sigma introduce the two Marvel villains his base, Sigma and Ultron initiates their sinister plan to betray Thanos, with Sigma had Ultron to stab his body and uploading his mind to the latter's body via Reality Stone, thus creating Ultron Sigma. After defeating Thanos and taking the Space Stone, Ultron Sigma finally transformed "their" old Ultron form with both Space and Reality Stones, and begin to use the two said stones to converge both Marvel and Capcom world timelines and realities into one.
At some point after the Convergences on his 88 days of reign, Ultron Sigma already overthrown a merged city of Asgard known as Xgard (a fusion between Marvel's Asgard and Capcom's Abel City's Highway), and formed another alliance with Grandmaster Meio, entrusting him a Power Stone to develop an ultimate weapon powered by Sigma virus and Power Stone itself called Gravitron Core, using the core as a bomb to infect across the globe when it falls successfully. Ultron Sigma and Grandmaster Meio also captured Zero and rewritten his core routine into Meio's brainwashed guards known as "Oblivion". 88 days, the heroes began to raid Xgard to restore the throne from Ultron Sigma. As Ultron Sigma comes out and taken out most of the heroes, Thor Odinson, alongside Rad Spencer, challenges Ultron Sigma for the throne of Xgard, with Ultron Sigma declaring arrogantly that their will is law and thus they will destroy them. Regardless of whether the player wins or loses, however, the first fight against Ultron Sigma will end the same way once Ultron Sigma's health is whittled down to a third: Ultron Sigma will disable Spencer and Thor with a chest blast and a beam saber slash, respectively, and then decries them as pitiful before asking if they even realize how powerless they truly were. They then steps on Thor's hand just as he reached for Mjolnir, and declared that they are not to weep for the Asgardians due to their achieving perfection under their rule. He then cruelly tells Thor that he'll soon "be their prince once more" before charging up an energy blast on his arm to transform him into a Xgardian. Before they could land the final blow, however, Ultron Sigma was shot in the back. Unphased by the gunshot, Ultron Sigma, realizing that Chris Redfield was behind him and shot him, calmly asks him if the commander had something to add in terms of great insight. Chris, however, merely tells them that they’re going down. Ultron Sigma expresses some mock-disappointment in that all they had to offer was a "banal cliché" before declaring it to be fitting as final words from them. Ultron Sigma then levitates up and proceeds to charge up an attack above them before firing at Chris. However, Captain America deflects the blast with his shield, while Strider Hiryu retrieves Thor and teleports behind, with Ultron Sigma noticing just as it happens. The heroes then escape via Doctor Strange's portal, although Ultron Sigma nonetheless are glad over the overall turn of events despite losing their quarry.
They soon found that the heroes had been trying to distract Ultron Sigma for Chris and Doctor Strange's rescue ops to rescue Thanos. In case if Thanos hasn't been found by "themselves", it is revealed that Ultron Sigma had been implanting an immature Sigma virus into Thor's body before Chris and Doctor Strange's ops arrived in time after successfully rescue Thanos, and Thanos alerts the heroes that they are too late to prevent the Sigma virus matured in Thor's body, thus the heroes lost their renowned Thunder God of Norse, now turned Ultron Sigma's most loyal puppet. As the heroes splits up under the tips from Thanos, Ultron Sigma had some of their Sigma-virus powered drones to trail Captain America and Chun-Li behind, as well as infecting Dah'ren Mohran to invade Valkanda (a merger nation between Marvel's Wakanda and Capcom's Val Habar) for Time Stone, which is currently under Black Panther's hand.
Despite their failed invasion on Valkanda, Ultron Sigma invades the Avengers Tower where Thanos is held, as Thanos calls out Ultron Sigma are no god, recalling what Thanos warned Ultron Sigma about understanding the full potential of using some of all 6 Infinity Stones prior Ultron fused with Sigma. Apparently, Ultron Sigma are ambushed by Iron Man, Doctor Light and Spencer, electrifies Ultron Sigma into weakened state, just as Thanos is released and defeats the fused robots. When Thanos punches Ultron Sigma’s head many times, his cracks the Reality Stone on Ultron Sigma’s head with his punch, causing Ultron Sigma to feel pain and retreat to repair the Reality Stone from completely cracked into a piece of shard separating both Ultron and Sigma, just as he already have Grandmaster Meio to successfully detonate the Sigma virus gas infused Core bomb into the Earth’s atmosphere, forcing the heroes and Thanos have no choice to destroy Ultron Sigma before they are completely infected by Sigma virus.
Eventually, Ultron Sigma gained the Soul Stone via Dante, who seemingly defected to their side. However, unbeknownst to them, Dante faked defection specifically to lure them into a trap: Specifically, the Soul Stone could only be wielded by someone with the noblest of souls, and Ultron Sigma, being a fusion of two mechanical life forms, lacked any soul at all, much less the noblest of souls. This ultimately resulted in Ultron Sigma being overwhelmed by its energy. Unfortunately, this didn't end him, as the Soul Stone has few side-effects like causing fused body transform into a monstrosity known as Ultron Omega, featuring Ultron's torso atop a giant Sigma head, who then promptly exacted revenge on Dante by trapping him in an energy field before the heroes (including the recent arrivals of Zero, Ryu, Gamora, Frank West, Doctor Strange, and Iron Man, but without Thanos) ultimately weakened them enough to not only release Dante, but also leave them vulnerable to X's Infinity Buster (empowered by the other fighters due to the sheer energy emitted from the Buster and Infinity Stones being such that even X with his limitless potential had trouble controlling the energy.).
Ultron Sigma is an extremely powerful individual, combining the abilities of his two already formidable fusees. From Sigma, they exhibits an extremely high level of combat skill, able to fight off and defeat the Avengers by themselves as well as X, thwarting his Nova Strike. In battle, they wields a beam saber similar to Sigma's, colored purple. With Ultron, they are able to generate beams or fields of energy from the palms of their hands. Already possessed of great strength, they was also able to lock blades with Dante and his Rebellion and ultimately fend him off with it. They also possesses high speed flight and the ability to teleport at will. Their greatest weapons however, are the Reality and Space Stones embedded in their body. With the Space Stone, Ultron Sigma can freely move between different dimensions and invade them at their leisure and with the Reality Stone, all facets of the universe itself are at their control, able to corrupt and transform entire worlds. They can even rip open in the space time continuum to summon a meteor shower above their targets. In-game, they can use Reality and Space Stones to fire slow moving projectiles or invoke the power of the elements as well as gravitate opponents toward him or imprison them in a prism of enclosed space. They are also able to infect groups of people with an advanced form of the Sigma Virus that is capable of infecting even organic beings like humans and used this ability to infect the entirety of Asgard's military force (referred to as Xgardians). This variation also manifests itself outwardly as a metallic material with glowing cybernetic patterns that spread across the body, subjugating them to Ultron Sigma's will. When not using the Xgardians, they can still employ Ultron Drones to attack in his stead.
Because Ultron Sigma has no soul to call his own, they unable to harness the full power of the Infinity Stones, let alone the Soul Stone, which caused a mutation of their merged frame, morphing him into Ultron Omega, with additional side-effects that cause their Sigma Virus negated and turned them insane. Since they are still a machine, they can also be short-circuited by a massive surge of electricity as shown when Dr. Light, Iron Man and Nathan Spencer channeled the Avengers' Tower's power supply to weaken the,. When Thanos cracked the Reality Stone by stomping on their head, this also caused the bond between their fusees to weaken, their form exhibiting visual glitches, prompting them to retreat for repairs.
- Ultron Sigma is named after the naming conventions used for both characters, as Sigma's bodies are treated like forms (Wolf Sigma, Neo Sigma, etc.) whereas Ultron's are treated more like models (Ultron-2, Ultron-5, etc.)
- Outside Story Mode via mod hack on PC version, Ultron Sigma can use Soul Stone.
- Ultron-Sigma can be considered to be a counterpart to the entity born from Rugal Bernstein (The King of Fighters series boss) when the character lost the control of his Orochi Power and the Satsui no Hado that he obtained from Akuma (Street Fighter series boss) in Capcom vs. SNK 2. The character exists in Rugal's body, but gains Akuma's influence and physical characteristics. Although unnamed, many fans call this entity as Toshin (闘神 Tōshin, literally "God of Fighting").
- He also can be considered a counterpart to Dark Kahn from Mortal Kombat vs. DC Universe, a fusion of Darkseid and Shao Kahn, two supervillains from two alternate universes and two different companies, and who spreads a negative influence, the Sigma virus, similar to how Dark Kahn spreads the Kombat Rage in Mortal Kombat vs. DC Universe. (Note: Midway Games, the original developer for the Mortal Kombat series of video games, was bought out by Warner Bros., which also owns DC Comics, and was rebranded as NetherRealm Studios.)