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Non-player characters seem each to contain within itself a representation of the entire universe from its unique perspective. Or they don't.
NPCs act as singular monads unto themselves, each with its own inner mental life and perspective on the game world. Or else they exist only within the shared imagined space of the game, brought to life through the creative minds of the players. If both: you get an illusion of autonomous existence whose roles and actions depend intrinsically on the desires and choices of the players.
So let, for every me, i, say: these non-player characters reflect the Leibnizian idea that all monads mirror and represent the whole of existence — save in their own limited vantage. Though imaginary, they take on, like charge, a reality within the pretense of the game. Their purpose becomes to engage and react to the player characters, eliciting storylines and experiences desired by the players.
A non-player character that serves as a contrast or complement to a player character — in order to highlight or bring forth certain qualities, desires, or tendencies within that player character — gets you a foil.