Created by Dr. Thomas Light in the year 20XX, X was the first free thinking robot. Dr. Light gave X his name because in mathematics X is a variable, representing what he described as X's "limitless potential". Dr. Light sealed him away because he was unsure of how the world would react to a robot with the ability to reason and think. X was only supposed to be sealed for diagnostics for a period of 30 years, but Dr. Light didn't live to see that day. Instead, X ended up in that capsule for a period of 100 years. He was found in 21XX by Dr. Cain, who discovered him in an archeological dig of Dr. Light's lab. Dr. Cain used X's designs to create a new race of robots that had free-will he called 'Reploids'.
Mega Man X Series
When Reploids began to go ' Maverick', crimes involving Reploids increased dramatically. Because of this, a special division of Reploids was created, the Maverick Hunters. X joined the Maverick Hunters, but often had trouble preforming his duties because he worried too much. But in the first game in the Mega Man X series, X's Maverick Hunter leader Sigma went Maverick and started a rebellion. It was then up to X, along with his friend and mentor Zero, to stop him. X goes about defeating Sigma's eight Maverick leaders, growing stronger and gaining new powers as he went along. When X and Zero met up at Sigma's fortress, Zero was killed by the Maverick Vile. X defeated Vile, and then went on to defeat Sigma.
In the sequel, Mega Man X2, X was facing a new trio of enemies called the X-Hunters. The X-Hunters essentially toyed with X, released Mavericks for X to fight in order to study and watch him. They sent him a message letting him know that they each had a piece of Zero's body, and that he would have to defeat them to get the pieces of Zero back. Mega Man X2 featured multiple story paths, but ultimately Zero is resembled and helps X defeat Sigma (who turned out to be behind the X-Hunters plot).
In the third game, Mega Man X3, the Maverick virus has been eliminated by a Reploid named Dr. Doppler. A utopia begins, only to be interrupted by a Maverick revolt by the Mavericks were supposed to be fixed. X, Zero, and the other Maverick Hunters are dispatched only to realize the revolt was a decoy to lure them away from HQ. X and Zero rush back to defend the base. Eventually, X realizes that Doppler had been under the influence of Sigma, who had returned once again. Zero and X were able to stop Sigma (once again), and learned that the reason he was able to come back was that Sigma had actually become a virus.
X4 set X against an organization known as Repliforce. Originally created to be a military Reploid organization to assist the Maverick Hunters, they went Maverick and started a war. X had to fight against them only to realize that Sigma had been behind the plot all along. X also learns that his assistant at Maverick Hunter HQ, Double, was actually an agent of Sigma. X is able to defeat Double, Repliforce, and Sigma. Afterwords, X calls Zero and begs him to destroy him if he should ever go Maverick. (The canon of this version of X4 is in question due to X and Zero both having alternative plotlines depending on who you play as.)
Mega Man X5 set X and Zero up against Sigma once again. This time, Sigma made his appearance right at the start of the game, being the very first boss of X5. When they defeat him, they realize he intended to lose, because afterwords a virus is spread all across Earth. Additionally, a Mercenary Reploid named Dynamo has been hired by Sigma to crash the space colony Eurasia into the Earth. The canon story progression of X5 is assumed as the following: Zero flies a space shuttle into Eurasia to stop it from crashing. He survived, and the pair went to face off against Sigma. After defeating Sigma there is a giant explosion. X finds Zero, in a very battered condition. He picks him up, but he then hears a voice behind him coming from a large skull face of Sigma. Sigma fires a laser straight through both X and Zero. Zero manages to blast Sigma with his arm canon, finally defeating him. Dr. Light arrives and saves X from dying. He lives on, leading the Maverick Hunters and carrying Zero's saber into battle with him.
Mega Man X6 takes place three weeks later. X is sent out to investigate the new "Nightmare Phenomena" and the accompanying appearance of a new Maverick known as the "Zero Nightmare." While rumored to be Zero in a new Maverick form, X defeats Nightmare Zero and learns that it was actually just a copy of Zero. The real Zero makes himself known, explaining that after the disaster of X5 he was forced to go into hiding to repair himself. X and Zero then go and defeat the creator of the Zero Nightmare, Gate, and Sigma (again).
X7 showed a very different side to X. X has moved to an HQ position rather than working out in the field. X did this because he began to have issues with violence and war. This leaves Zero to do the work on his own, and while out on a mission he runs into a Reploid named Axl, who was formerly a member of the group Red Alert. Red Alert is a group that is similar to the Maverick Hunters, in that they hunt Mavericks, but they use questionable methods. Axl leaves them and wants to join Zero and the Maverick Hunters in stopping them. X doesn't want Axl to join the Maverick Hunters, but Zero goes ahead and lets him join him in stopping Red Alert. X rejoins them to fight Sigma (again). Back at HQ, X continues to refuse Axl joining the Maverick Hunters, but Axl ends up joining up anyways.
X8 kicks off with X, Zero, and Axl investigating a crab-like Maverick (X's personality change in X7 is pretty much ignored by the game). Unfortunately, they are interrupted by X and Zero's old nemesis and former Maverick Hunter Vile, who kidnaps a Reploid known as Lumine. Lumine is the leader of the Jakob project, a project by the humans to build an elevator to the moon. Sigma also returns (yes, again) and it becomes apparent that he was actually behind Vile's kidnapping of Lumine. X, Zero and Axl are able to defeat Sigma. Lumine then reveals that it was actually him who was the mastermind behind the entire plot, though they are able to defeat him as well. As they leave, Zero gives X a sort of "pep-talk", to ignore some of things Lumine had said about their race (because X was shaken by the encounter and some of the things Lumine said).
Mega Man Zero Series
X next appears (chronologically) in the Mega Man Zero series on the Game Boy Advance as a Cyber Elf. X is only a supporting character, assisting Zero when he can due to no longer having use of his physical body, which was destroyed. He now has a new doppelganger, Copy X, who inhabits a similar body to the original X.
He finally departs Zero's assistance in Mega Man Zero 3, after seeing the fall of the Dark Elf, Copy X, and Omega Zero.
Mega Man ZX Series
X later appears in the ZX series, which takes place several hundred years after the Zero series. He appears here as Biometal X, which is essentially an armor with his soul sealed inside that helps the protagonists, Vent and Aile, in their battles by granting them his abilities.
Mega Man X Armors
A key feature to Mega Man X is the fact he can change armors. They give him special abilities like flight, a better x buster, and the ability to stick to ceilings. Here is a breakdown of some of the armors.
The ultimate armor is the best armor in a Mega Man X4-X6. It has the most powerful X Buster and the most all around fighting capabilities.
Force armor is found in Mega Man X4 and has an upgraded Buster that can be equipped to either shoot one powerful charge shot or store four weaker charge shots to shoot at any time. Also, this armor has the ability to hover for a short period of time.
Falcon Armor appears in both Mega Man X5 and X6. In Mega Man X5 This armor has a weaker charge shot, but has the ability to fly for an extended period of time (which is pretty awesome). In X6 the armor loses it's flight ability and has a stronger charge shot than it's original release in X5. Also, X obtains Zero's sabre.
The Blade Armor is featured in Mega Man X6 and has a focus on mobility. The dash can go farther, do damage, and X can dash up and down. The charge shot is weaker, but you can charge the Z-sabre. Lastly, while wearing this armor X can cling to walls and not slide down them as he is clinging to them.
The Shadow Armor is featured in Mega Man X6 and relies on maneuverability once again. The Shadow Armor can shoot shurikens, cling to ceilings, and swing the Z-sabre faster. The downside was this Armor could not air dash and the buster shots would randomly veer off into different directions. This armor was mostly used to make X more capable with the Z-sabre.
The Gaea Armor has improvements in offense and defense however, the armor lacks speed and maneuverability. The charged shot does immense damage, but the armor can not use special weapons. The Gaea Armor can cling to walls indefinitely and does not get killed by spikes. This Armor also loses the ability to air dash and is slower.
The Hermes Armor is featured in Mega Man X8 and has one main feature. That feature is X can boost all of his stats with this armor for a short period of time.
The Icarus Armor is also featured in Mega Man X8 and, again only has one new feature. This feature is the ability to perform a giga attack similar to that found in Mega Man X6. This attack can performed repeatedly, but if not charged the attack does less damage and has a smaller range.
The first and only armor you find in Mega Man X. This armor has an improved X buster and defense. X can also power up to a third level charge shot for special weapons. Lastly X can jump into certain blocks and destroy them.
This Armor appears in both Mega Man X2 and X3 it has distinct qualities in both games, however they both look the same. In X2 the Armor can shoot two charged shots, have level three charges for special weapons, air dash and dash jump, and see hidden passages that could been seen without the helmet part. Also as a funny side not, With full armor X can perform a shoryuken like Ryu and Ken from Street Fighter. In X3 the abilities vary slightly X can air dash, but he can air dash up as well as air dash twice , he can have a four level charge shot for his buster and have two busters, he can take up to 75% less damage, and lastly, his armor can turn gold.