When a character In a show, musical, play, book, movie, etc. is mentioned or seen for a very short period of time, having nearly no impact on the story, but is generally either loved or hated by the entire fandom. Named after Peggy Schuyler, a character from the musical Hamilton.
Because Peggy Schuyler only shows up in one song, yet is seen as an “UwU sawft cinnamon roll” and the majority of the fandom loves her, she is a prime example of The Peggy Effect.