stickler

One who insists on something unyieldingly.

Something puzzling or difficult.

An attendant on or a judge of a contest, as a duel; a second; hence, an arbitrator; a peacemaker.

An obstinate contender about anything, often about a thing of little consequence: as, a stickler for ceremony; an advocate; a partizan.

One who stickles.

One who arbitrates a duel; a sidesman to a fencer; a second; an umpire.

One who pertinaciously contends for some trifling things, as a point of etiquette; an unreasonable, obstinate contender.

someone who insists on something