Well, as far as I can tell we don't have a "core identity" in the sense of the person born into the body. As far as I can tell she is gone. The rest of us came later. If however you mean "person who fronts/has the body most often" then that would probably be me. I figured out the whole Multiplicity thing about 6 years ago, when chatting to someone online who was also Multiple, and realising that their experiences matched mine in many ways. I then chatted to a few more and found more similarities. Prior to that, I had taken a "new-age" kind of approach, and believed that these voices I was talking to were channelled spirits or something like that. Even now, I still think the line is very blurred. Some of us really are channelled spirits.

As for "keeping track of everyone" it's not really any different from keeping track of external friends. The ones I spend most time with are the ones I know the best.