From my understanding of the term, I don't think apostrophe is the right word you want- it's an address to the personification of an abstract concept or imaginary object, like Death or the dagger in Macbeth. So it wouldn't apply to a real person *
In terms of plays directed at a main character who never appears, the closest I can think of is Jean Cocteau's "The Human Voice" which is a monologue piece- it's a telephone conversation; we never see or hear the man on the other end.
Personally, I would be more inclined to think of the character(s) who actually appear on the stage as the main characters, as they tend to get more development. The Ghost
is as Characters as Device
as they come.
Dramaturg, troper, theatre reviewer. (Please hire me.)