Guest character
In serialized works of art, such as TV serials, games, etc., a guest character is a character which acts only in a few episodes or scenes, unlike regular characters (both core and secondary ones), which act throughout the whole or a significant duration of the series.
Unlike regular characters, the guest ones do not need to be carefully incorporated into the story line with all its ramifications: they create a piece of drama and then disappear without consequences to the narrative structure, unlike core characters, for which any significant conflict must be traced during a considerable time, which is often seen as an unjustified waste of resources. [1]
There may also be a continuing or recurring guest character .[2]
Sometimes a guest character may gain popularity and turn into a regular one. [3]