THOMAS LIGHT
In the year 21XX, Doctor Light, or otherwise known as Thomas Light created X, with the name "X" representing a variable, and therefore, infinite potential. Thus, X was given free will and human-like thought, being the first ever robot to have a consciousness. To make sure that X would forever shield humanity from harm instead of doing otherwise, Thomas Light sealed him away in a capsule for the next 30 years in order to train his electronic brain, leaving a message behind explaining his vision, knowing he would not live to see the future 30 years.
In parallel to Doctor Thomas Light, Doctor Albert W. Wily, Thomas' colleague, quickly became jealous of Thomas' achievement of creating the first ever conscious robot. And so, Doctor Wily stole the plans for X and created ZERO, better, stronger, faster, and smarter than X, Dr. Wily calling him "My greatest creation!" Being as old as Light, he too sealed ZERO away in a time capsule, wishing that one day someone would open his Pandora's Box and close it before the hope came out, but he would not live to see that day.
THE NATURE OF X AND ZERO
ZERO was designed to purely be a killing machine - a stark contrast to X who was designed to be humanity's champion, reflecting the deepest human strengths of innocence and a strong-feeling heart, hence X was built to resemble a teenage boy. ZERO was built to resemble a young adult, likely a young man in his early twenties, as he is taller and bulkier than X, not built like the typical villain archetype, however his true strength comes from the opposing indomitable human spirit that X does not possess, that being the SIGMA Virus that Dr.Wily had engineered. The SIGMA Virus imbues ZERO with the indomitable human spirit, filling him with a terrifying sense of determination to persevere and overcome that which would threaten him in any way. Thus, X embodies the human nature of self-sacrifice whilst ZERO embodies human determination. Ultimately, X and ZERO are the two clashing sides that co-exist within the human soul, being direct extensions of their creators.
Even though the nature of X and ZERO are opposed, they cancel each other out to form what we call a human. As a variable, X can go high up into infinity, however anything that encounters ZERO is reduced to ZERO, and so, this anomaly which occurs within mathematics and reality by extension is what creates the metaphysical - the human soul.
DOCTOR CAIN
In the XXII century, Doctor Cain, a scientist akin to Light and Wily, discovered during an archeological search, the ruins of Doctor Light and Wily's laboratories, alongside their respective creations : X and ZERO.
Following this incredible discovery, Doctor Cain proceeded to awaken the two, but the two machines reacted very differently. Once awakened, ZERO became extremely volatile and aggressive, whilst X was obedient and willing to cooperate with Doctor Cain's research team.
Doctor Cain eagerly began to research X's core technology, but himself not being quite able to decipher it fully, and in consequence, he created the REPLOIDS, bio-mechanical robots with their own personal traits and awoken consciousness, all created using X's blueprints and technology.
THE REPLOIDS
The reploids created by Doctor Cain perfectly resembled humans, and animals alike, built to be able to think for themselves and make their own decisions, and so, they were all tasked with defending and aiding mankind. However, reploids often had their electronic brains corrupted, thus becoming MAVERICKS. MAVERICKS were aggressive reploids that became aggressive, seeking to destroy everything in their path. This happened because Doctor Cain could not entirely replicate X's gentle nature and nearly-human heart, thus leaving the rest of the reploids vulnerable to this new anomaly.
And so, a police force amongst the reploids named "MAVERICK HUNTERS" was created, composed of warriors and battle-ready reploids in order to combat the growing numbers of reploids going rogue. Amongst these MAVERICK hunters, there was SIGMA. SIGMA was the commander of the Maverick hunters, and soon enough, he came across a particularly powerful Maverick - ZERO.
SIGMA VS ZERO
SIGMA stood up to face ZERO, and in a fierce duel that followed, ZERO's MAVERICK VIRUS that he was created with was transferred to SIGMA, thus becoming the "SIGMA VIRUS". SIGMA retreated after ZERO fell unconscious due to the virus leaving his body, and he was brought back for research, and eventually, ZERO became a MAVERICK HUNTER himself, and the deuteragonist of the MEGAMAN X series, as X's best friend.
THE DAY OF SIGMA
SIGMA eventually became a Maverick, one of the most powerful, as a matter of fact, and from then on out, he became the antagonist of the MEGAMAN X series, being the one entity orchestrating what bullshit X and ZERO would have to go through in the following years.
And driven by SIGMA and his spread of the SIGMA virus, the battle between MAVERICKS and MAVERICK HUNTERS continued over the games MEGAMAN X, X2, X3, with both X and ZERO becoming legendary Maverick hunters and mythical symbols of the just nature of reploids.