Birthmate

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A birthmate is a type of headmate who's been a part of a system of which has existed since birth or cannot remember when or how they've formed. Typically this'll be an innate system or be protogenic, but not always. The term refers specifically to one of the said type of headmates of which has existed since the body's birth or for as long as anyone in the system can remember, rather than having been developed sometime afterward. Some systems have several headmates since birth, but may also get newer ones later on who, unlike birthmates, were not always there since the beginning of the body's life. This term helps differentiate the first headmates from the newer ones.

Related Terms
The core is a headmate with a direct connection to the body's pre-plural self.

There is also original, an original is a headmate that has existed before anyone else has.

An Initial headmate is one that was not split off any other headmate and don't have a clear moment of creation. They have "always been there" in some way.