The Non-Player Character, in the world of role playing games, is a character who is controlled by the game master or one or more players for the purposes of furthering the plot, but who is (most often) not portrayed with their own agency.
They are basically a “placeholder” that fits the needs of the other characters, or of the plot.
I’m going to expand the definition a bit, because I think it is a useful metaphor.
Many of us approach the other people in our lives as NPCs.
Here is a rather horrifying and creepy story that I came across, from “Controlling People” by Patricia Evans, that illustrates one such relationship between a “game master” and their “NPC” as it might play out in real life:
What is fascinating to me about some portrayals in the media of women who are excessively adorable but lack inner life or agency (I’ll elaborate on this later because I have my own interpretation and take on the “Manic Pixie Dream Girl” stock character) is how they are basically serving the purpose of being an NPC to the main character and to some degree, the “adorableness” may be perceived via an unreliable narrator who really does not know his love interest’s inner life. He has to idealize her via what little he does know – her various quirks and her music preferences.
You also frequently see this portrayal of the male costar in a “chick flick”; he doesn’t really exist outside of the female character’s mind. In fact, I’d love to see a film that actually played with this concept and subverted it (he in fact DOESN’T exist outside of her mind).
The thing is, we act out these dramas in real life. Many of us never get past seeing other people as NPCs.