Reploids

Reploids are a race of sentient androids with complete free will and thought processes comparable to those of humans.

Characteristics
Unlike Mechaniloids, whose artificial intelligence is bound to their programming, Reploids are the same as humans on a mental level; they think and feel, giving them the ability to make their own decisions and develop unique personalities. Although Reploids undergo major physical changes throughout the series, they generally retain this one trait.

Reploids are humanoids, and may appear either male or female. They usually have a human face (and sometimes, hair), as well as bulging armor around the forelimbs and upper body. However, many are built to actually resemble robotic animals with humanoid traits. These particular Reploids tend to be designed for very specific tasks; for example, a heavy duty Reploid may be large and bulky, whereas a marine Reploid may be based on a fish. There is no limit to the tasks Reploids can perform with the right design.

Reploids have a form of DNA. As machines, Reploids do not age, although they still require rest and maintenance to remain functional.

Origins
X was the first fully free-thinking robot of his kind. While created before the term Reploid and being the original robot the Reploids were based on, X is considered a Reploid nonetheless as the term is for robots that think like humans, and X also refers to himself as such. X also remains more advanced than Reploids for hundreds of years, despite Reploids being based upon X's template. This is because X has as close to a human conscience and cognition as possible, a remarkable trait that, like other aspects of X's enigmatic design, proved extremely difficult to reverse-engineer, and thus for generations Reploids were not quite able to think and feel as deeply and humanly as X. Furthermore, X is able to think about and develop his conscience and morality to a degree that comes naturally to humans but that is a struggle for Reploids, and perhaps unsurprisingly X is also extremely resilient to viruses which, whilst more effective in their ability to control less advanced robots, are nonetheless able to twist the thinking, empathy, and even sanity of Reploids other than the most advanced.

Mega Man X Chronicles
In 21XX, some 100 years after the apparent death of Doctor Thomas Light, an archaeologist named Doctor Cain excavated one of Doctor Light's laboratories and discovered a capsule which contained the highly advanced robot named "X". X represented a substantial jump in robotics technology, prompting Cain to reproduce X's technology from Light's original work and notes. Being unable to reproduce the technology perfectly, Cain was forced to invent alternative solutions. All robotic designs based on X's technology have been referred to as "Reploids," since they are "replicas" of humans.

Thenceforth, Reploids were mass-produced to live among humans on Earth. However, some Reploids started to rebel against their creators, and would harm those they did not agree with. These reploids were labeled as Mavericks. A council decided to create a group of reploids that would rise to the occasion of Maverick outbreaks. This group was dubbed the Maverick Hunters.

However, a few of these Maverick Hunters would also break away from their original programming and cause, after the climax of Sigma's first encounter with a rogue Zero. From then on, a mysterious virus (that would be later be known as the infamous Sigma Virus) would infect this Reploid, who would cause mass destruction and trigger the Maverick Wars; for decades, the conflict divided the Reploids between those who fought for the humans and those who opposed them; either because of viral infection, rejection of their status as tools of the humans or other reasons.