truther

noun4

1. •
   A conspiracy theorist who does not believe the accepted story about some significant event, fact, etc.

   • For instance, "truthers" who believe in a US government conspiracy to destroy the pentagon — 2013
2. •
   (informal) Someone who believes that the US government was responsible for the 9/11 attacks.
3. •
   (US, informal) Someone who tells the truth.

   • A liar can't begin to start the riots a truther can. — 1915
4. •
   (slang, humorous) An advocate, especially for a headcanon.