Mysterion

Mysterion is a character in the animated TV show South Park and featured in the episode, The Coon.

She/he is a student at South Park Elementary in Mr. Garrison's class who hides behind a pseudonym, 'Mysterion' and was the rival to The Coon. S/he unveiled himself after 'The Coon' persuaded him/her to for the good of the town.

Due to the South Park animation style it is unclear who it was except Cartman said 'I knew it, I even said it was you before.'