Not sure how it happened within us, but as growing up early on we had a one that i guess was created just to keep us coping and out of harms way.. That one stays in the head, but never really takes over the shell.
Very hard for me to explain, but she's like a book that we all need when something comes up and we need to know something to answer or need to go and make sure things are turned off. or doors are locked, or anything like that... she coexists with most all of us, there are still a few that none of us have met, and she isn't able to coexist with them for reasons unknown. but that only happens in short periods and not very often..