Mega Man X (Marvel vs. Capcom series)

Mega Man X is a is a playable character that made his appearance in Marvel vs. Capcom: Infinite. Though he made his technical debut in Marvel vs. Capcom 3: Fate of Two Worlds, but only as a downloadable costume for Zero and as a card in the Heroes and Heralds Mode. Though his moveset and voice did not change, X did share some of Zero's abilities and weapons during a few points throughout the series, namely his Z-Saber.

Marvel vs. Capcom 3: Fate of Two Worlds
Mega Man X made an appreance as a card in the Heroes and Heralds Mode. He was later confirmed as a downloadable costumes for both Zero and and Frank West and was released on February 7, 2012.

Marvel vs. Capcom: Infinite
Mega Man X, along with Strider, Captain Marvel, Gamora, Nova & Rocket head to Knowmoon to retrieve the Power Stone from Grandmaster Meio. While looking for it, X witnesses Zero being mind-controlled by Meio and is forced to fight him. X wins the fight and Zero regains his consciousness and allies himself with the rest of the heroes from then on while Strider and Gamora deliver the finishing blow on Meio.

X's arm cannon design was used by Iron Man to try and create the Infinity Buster to take down Ultron Sigma. X goes with a number of other heroes back to Xgard to keep Ultron Sigma busy while Iron Man and Thanos finish making the Infinity Buster. When Thor is freed from mind control due to the Soul Stone, he and X team up to weaken Ultron Omega, alongside Captain Marvel and Dante. Iron Man then arrives to summon the Infinity Buster. X is unable to hold the strength of the buster even when powered up, however with the assistance of Doctor Strange and Morrigan, X was able to be strong enough to fire the Infinity Buster and destroy Ultron Omega, ending the monstrous dictator's rule.