Which make me curious how Parvati's personality going to be like. Consider Sakura's... *Ahem* body condition and all that.
well she is an all loving mother figure in hindu myth, and seeing how possession in Fate seems to overwrite the base personality with the spirit... she might be Raiko-level lovingly motherly.
Being in or a friend of the Emiya family is the leading cause of becoming a Servant.
Eh, Nasu-multiverse may not be infinite, but it's the next best thing. For any given individual, there's probably a timeline somewhere in the Kaleidoscope where they did something to earn a place on the Throne of Heroes.
It's not that versatile, actually, (Kerry for example can't ever happen except in a history that will never be) but the pseudo-servant thing makes it really easy to servant up pretty much anybody who relates to a HGW in some capacity.