Answer: Harley Quinn

Harley Quinn is a character in the Batman universe who was first introduced in September of 1992 via Batman: The Animated Series. She was intended to be a single-appearance character, a temporary side-kick for the Joker during a single episode. Specifically, the series episode “Joker’s Favor” originally called for the Joker to jump out of a cake and take someone hostage. The writers thought it seemed out of place to have the Joker jump from the cake so they created the Harley Quinn sidekick character to do so. After the character’s creation, the episode was further redrafted and, ironically, the Joker ended up jumping out of the cake after all.

Harley Quinn’s character proved to be so popular that the writer’s kept her on as a permanent sidekick for the Joker in The Animated Series. Her presence in the television show proved so enduringly popular that Quinn was later added to the actual Batman comic book canon as an official character in 1999. Between 2001 and 2003 there was even an entire comic book series devoted to her, a series rebooted in 2013 for a second release. Not a bad run for a gig that started as what amounted to a walk-on role.