When someone with a Strong heart becomes a Heartless, a Nobody is born
Complete Being = Heart, Body, and Soul
Heartless = Heart taken and consumed by darkness, possible to retain human Form and memories when becoming one, but Xehanort is the only one to have done so as far as we know
Nobody = Made form the Body and Soul left behind when the Heartless was created, sent to another realm where it becomes a Nobody. With an exceptionally strong Heart, a Shell is made, where the Nobody retains human form with similar appearance and memories from when it was whole