This article is from the Dissociation FAQ, by Discord (firstname.lastname@example.org) with numerous contributions by others.
The birth person is the alter that was present from the beginning,
the one born into the body, who was present before the multiple
system was created. Some multiples believe they were born multiple.
In any event, the birth person may be the host (although certainly
not always) and/or the main alter (again, certainly not always) and
therefore often goes by the name of the body, at least officially.
The birth person may be still at the age where the original split
occurred, having missed all the time in the interim. The birth person
may not be accessible to those alters who are generally out.