Homunculus

In alchemy, Homunculus (ホムンクルス) is a creature given birth from the combination of human sperm and multiple substances/elements without the use of a womb. The result of not being born through normal methods means short stature, shorter life, and lacking in some parts of intellect. Reproduction functions are also gone. Not quite a human created with circuits, but rather, circuits that were born like a human being. Nature given a human body. Fragile existences, but they have power superior to that of human beings.

The concept of aging doesn't exist among them. Once born, the necessary knowledge and reason for existence are taken and imputed by nature. A perfect homunculus is an extension of nature created by the hands of man and as long as the planet's still alive, they will not die from old age. They will die if destroyed and in exchange for their potential longevity, are weak as lifeforms.

The Einzbern homunculi, even ones deemed a failure, are more powerful than magi and can even be a threat to an agent from the Association. Models based on Justica (who was a homunculus herself), such as Irisviel and Ilya share the memories and personality of the original. In a way, they're the same being.

Unlike normal homunculi, Ilya has aged normally up until a certain point. Also, unlike others, she apparently does have a life span. After any of the routes where she survives, she has only one year left to live.

Also, homunculi bodies are apparently made from ether (at the very least, Leysritt is made from ether it seems).