Spawner

A unknown person you have never seen before, often almost appearing to have "spawned" into existence. A spawner has no defining traits, and often are completely forgettable. Spawners behave in a very similar fashion to video game NPCs, in that they appear out of thin air and rarely possess a personality that isn't surface-level.

Person A: Who is that guy? Never seen him before.
Person B: I have no idea. He's a total spawner.