Step 1: Understand the meaning of "protagonist" - the main character or hero of a story, the one the reader supports.
Step 2: An antonym must express the opposite idea - the character who opposes or works against the protagonist.
Step 3: Among the given options, only "antagonist" directly means the opponent or rival of the protagonist.
Step 4: The remaining words are unrelated in meaning: agnostic (one uncertain about God's existence), arsonist (one who commits arson), anarchist (one who opposes organized government).
Final Answer: \(\boxed{\text{antagonist}}\)