In 2017, Diana Gabaldon, an author and friend of the writer George R. R. Martin, described a conversation with Martin where he mentioned that one reason he was struggling with to write The Winds of Winter (the long-awaited sixth instalment of his A Song of Ice and Fire series of fantasy novels) was that there was a character that he later regretted killing, saying ‘You ever kill somebody off that you later realised you needed?’ https://old.reddit.com/r/asoiaf/comments/8rumol/spoiler_main_diana_gabaldon_author_of_outlander/?embed_host_url=https%3A%2F%2Fwikiofthrones.com%2Fgeorge-r-r-martin-delay-the-winds-of-winter-killed-important-character)

Since then, fans of the series have speculated about who this character might be, and have put forward numerous theories:

https://old.reddit.com/r/asoiaf/comments/9kcxxs/spoilers_extended_who_was_the_character_that_grrm/

https://old.reddit.com/r/asoiaf/comments/1azzixi/spoilers_extended_who_was_the_character_martin/

https://www.youtube.com/watch?v=10Yr0JHyROQ

This market resolves to the identity of that character, if it is ever revealed by Martin (it will resolve N/A after Martin's death if it looks like we will never know the answer). I've added some popular theories, but anyone can add a character.