Definitions of trier
someone who always makes an effort, even if they might not be successful
As a player, he was a real trier. He wasn't the most talented player, but would always give 110 per cent.
the person (such as a judge) or group of people (such as a jury) who determines either the facts (=the trier of fact) or the law (=the trier of law) in a lawsuit
If there is no jury the judge is the trier of fact as well as the trier of law.