Someone who insistently advocates for something, especially for strict observance of a formal rule.
A referee or adjudicator at a fight, wrestling match, duel, etc. who ensures fair play.
A strict follower of a doctrine.
One who believes dogmatically in something regardless of evidence or even conclusive proof that the thing is false or was staged; one who has true-believer syndrome.
Used other than figuratively or idiomatically: see true, believer.