Subpersonalities may be defined as little, semi-autonomous clusters of traits inside us with their own peculiar thoughts and worldviews that we as a whole employ in our daily lives. I personally think they overlap a lot with imaginary characters, which is why I just now typed "subpersonalities" into Google and came up with this blog entry from Integral Options Cafe, in which the author tries to define subpersonalities. The author also discusses some books and psychological theories that use the subpersonalities model. The author mentions the book Subpersonalities by John Rowan [which I have read and reviewed in my ongoing bibliography about paracosms and imaginary characters and such], as well as other sources that I have not looked into and need to.
Hooray, more places to look for interpretations of paracosms and imaginary characters!
Later I should write something about Internal Family Systems [TM].