Ultron Sigma is the merged form of Ultron and Sigma that appears as an antagonist in Marvel vs. Capcom: Infinite. He is voiced by Jim Meskimen and Chris Tergliafera, the voice actors for Ultron and Sigma, respectively. His "voice" is that of both Ultron and Sigma's voices overlapping each other, owing to his 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 his namesakes; featuring Ultron's smooth and futuristic textures, yet possessing Sigma's general bulky body shape spike-studded limbs and red cape, his overall color scheme being black and gold. His head is identical to Ultron's but exhibits a purple-ish glow as well as lining around his 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 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. He then steps on Thor's hand just as he reached for Mjolnir, and declared that he is not to weep for the Asgardians due to their achieving perfection under his 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 he 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 him that he's going down. Ultron Sigma expresses some mock-disappointment in that all he had to offer was a "banal cliché" before declaring it to be fitting as final words from him. Ultron Sigma then levitates up and proceeds to charge up an attack above him before firing at Chris Redfield. 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 is glad over the overall turn of events despite losing his quarry.
He 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 his 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 punch cracks the Reality Stone, 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 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 his side. However, unbeknownst to him, Dante faked defection specifically to lure him 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 fused body instead created 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, Doctor Strange, and Iron Man) ultimately weakened him enough to not only release Dante, but also leave him 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, he exhibits an extremely high level of combat skill, able to fight off and defeat the Avengers by himself as well as X, thwarting his Nova Strike.. In battle, he wields a beam saber similar to Sigma's, colored purple. With Ultron, he is able to generate beams or fields of energy from the palms of his hands. Already possessed of great strength, he was also able to lock blades with Dante and his Rebellion and ultimately fend him off with it. He also possesses high speed flight and the ability to teleport at will. His greatest weapons however, are the Reality and Space Stones embedded in his body. With the Space Stone, Ultron Sigma can freely move between different dimensions and invade them at his leisure and with the Reality Stone, all facets of the universe itself are at his control, able to corrupt and transform entire worlds. He can even rip open in the space time continuum to summon a meteor shower above his targets. In-game, he 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. He is 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, he can still employ Ultron Drones to attack in his stead.
Because Ultron Sigma has no soul to call his own, he is unable to harness the full power of the Infinity Stones, let alone the Soul Stone, which caused a mutation of his merged frame, morphing him into Ultron Omega. Since he is still a machine, he 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 him. When Thanos cracked the Reality Stone by stomping on his head, this also caused the bond between his fusees to weaken, his form exhibiting visual glitches, prompting him 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.)