A Machine Body is a mechanical frame used by sentient life forms to inhabit for the purpose of gaining immortality. A person with a Machine Body is referred to as a Machine Man.
The process of Mechanization was developed on La Metal after its artificial sun began to fade. To combat the cold conditions, Queen Prometheum put into motion her husband Dr. Ban's plan to provide La Metal's citizens with Machine Bodies that would not require food or warmth to function. However, many of the converted civilians lost their emotions in the process, becoming mindless slaves. An organized resistance soon developed to the domination of the rising Machine Empire. However, even after the Machine Empire was driven back, Machine Bodies and the knowledge of their manufacture continued to spread throughout the galaxies.
Machine Bodies are primarily bipedal and they range in appearance from a clunky robotic form with just a single large sensor for sight and electric speakers for speech, to looking almost indistinguishable from a regular human. Thanks to ongoing advancements in design, some Machine Bodies can function just as well if not better than a regular human body. Other Machine Bodies made from cheaper and unstable parts, however, are less functional.
Nonetheless the primary component is undoubtedly the brain holder which (depending on the version of the story) either houses the original physical brain of the body's owner, or else stores the data contained in the owner's old brain. As long as this device remains intact, the owner can replace body parts indefinitely or aquire whole new bodies. However, some brain holders cut off parts of the personality such as particular emotions or even general arousal and motivation due to certain issues.
It is possible, as a result of a brain holder's limitations, for a person in a machine body to become a ruthless killing machine or even just become a living puppet with no free will.
Even when brain holders are fully functional, due to their virtual immortality owners of Machine Bodies can come to feel a sense of superiority, often committing heinous acts such as hunting down regular humans for sport. (They generally face little hinderance from the law should they choose to do so - the "hunters" who killed Tetsuro's mother were technically vigilantes shooting violators of a dusk-to-dawn curfew that the local authorities had imposed on regular humans.)
Due to the great cost of Machine Bodies, owners are often nobles or of high class, which in their eyes provides further proof of their complete superiority. As such they are the envy of poorer classes that barely have enough resources to get by in life. All the same, the less fortunate do still often vie for machine bodies, and sometimes muster enough to purchase cheaper and more affordable alternatives to the more expensive parts. Unfortunately, these low-grade cybernetics are often the doom of these people.
There exists a very small cohort of people, however, who seek Machine Bodies only in cases of medical emergency. Such individuals are never shown to develop the inflated egos or signs of madness that so often plague those who turned to Mechanization out of pride, vanity, or hubris.