Carbon

Carbon, known as Decoy (デコイ) in Japan, is how the System refers to the artificially created humans that live on Terra (Earth). They are also referred to as Betas by the Master of Elysium.

Characteristics
Carbons are essentially "artificial humans," and the question of what they are and what their purpose is a major theme to series' elements of mystery. Like normal humans, they are organic beings made of flesh, but are able to replace their regular body parts by mechanical implants, ranging from simple arm replacements to full-body overhauls. Despite this, Carbons are at least partially if not entirely robotic. Many Carbons have mechanical portions of their bodies, some as simple as replacement parts and "patches" while notable others like MegaMan, Teisel and Glyde have almost entirely mechanical looking bodies. Additionally, MegaMan can switch out his arms for different weapons. Curiously, it seems exceptionally rare for female Carbons to have any mechanical parts, suggesting that it may be considered unattractive. (Early character designs for Legends show a nurse with a buster-like syringe on her arm, which the developers deemed frightening.)

Aside from similar biological actions, Carbons also behave more or less like humans. They are prone to building up societies, and have distinct cultures and beliefs varying by location. Carbons seem more or less ignorant of the ancients who came before them, and in many cases revere them as myth. They also make use of technology left behind by the ancients.

Reinitialization
A recurring theme in the Legends series involves awakened ancients attempting to "reinitialize" Carbons, i.e. wipe them out en masse. When MegaMan Juno is awakened, he explains that it's his job to routinely reinitialize Kattelox Island to keep the Carbons living on it from overpopulating.

Because of how Carbons are treated by the ancients, it's speculated that Carbons were originally created as a means to see if the world would be habitable by humans again, following whatever disaster occurred thousands of years in the past. The games have yet to offer a complete explanation, however.