Filler character
A Filler character is a character who is considered less useful to a given plot or story, there to take up space and make a world feel more full.
In some larps with pre-written characters, if they show up unintentionally, it can be considered a sign of poor writing; when each character is meant to have a perspective that tells a worthwhile story for each player, playing a filler character often means spending time at a larp left out of major plot beats and information, or unable to contribute or exert agency to the same extent as others.
There are also times when filler characters have their positive uses. In a longer game when a character meets their death partway through, particularly if it is a more spectacular or fulfilling moment, a player may pick up a filler character afterwards to be able to keep participating whilst also intentionally stepping back and giving other player characters the spotlight. Some players also intentionally design a character to be filler for someone else's story from the start, if they're planning to support another player or simply believe it will be a fun way to pass the time.