[Zexion smiled, though it's not an especially nice one, more self-deprecating than anything.]

Let's just say it was due to experimentation on hearts. When a strong-willed person loses their heart, it creates two beings: a Heartless, a weak shadowy creature, and a Nobody. The person's strength determines how strong the Nobody is. Some become low-ranking, weak Nobodies. A very few are like myself and retain their personality and sense of self.

[It is sort of a death in a way, just not a permanent one necessarily.]