K.J. points to the excellent Layman's Guide to Multiplicity, a great resource on what's sometimes referred to as Dissociative Identity Disorder and used to be called Multiple Personality Disorder--but many plurals don't consider themselves to have a disorder. See also the Astraea's Web Multiple Personality Glossary for preferred terms, including definitions of "plural" and "multiple" in this context.
(And because I can read my readers' minds at a distance, I'm pretty sure that at least eighteen of you are already, before you even finish reading the preceding paragraph, preparing to recommend that I read Set This House in Order. Which is one of those books that's been recommended to me by so many people that I'm going to have to read it sooner or later. (And the facts that it's by Matt Ruff and won a Tiptree Award make me even more interested in reading it.) But given that it took me about ten years to read Tigana after people started recommending it to me, I recommend not holding y'all's collective breath.)
I'm especially intrigued by the term "walk-in"; a concept I had encountered before, but I didn't know a name for it.