No surprises here. The narrator is the guy with the heroic quest He wants to prove that the red room isn’t haunted by spending the night in the room. He and his trusty candles represent the forces of light pitched against the forces of darkness. We have numerous such forces: the ghost that supposedly haunts the red room, the fear that actually haunts it, and the literal darkness. He’s also the central character, the only character we know much about, and the character around whom the plot centers.
Actually, you could make the case that the protagonist is not just the narrator, but also the narrator’s reason. Fear, which is definitely the antagonist, is something within the narrator. The narrator's fear struggles against his reason. It’s the narrator’s reason that gets "crushed" (45) by the darkness. Once this happens, he goes bananas and knocks himself unconscious.
That might be making things a little abstract, and it might not work so well in other respects: the narrator’s literal fight against the darkness with candles is certainly his own attempt to control his fear, but it’s not obvious how you could say his reason is fighting against the darkness. On the other hand, the narrator clearly identifies with his reason over his irrational fear. By making the narrator’s reason the protagonist (or one of the protagonists), you can frame the story as an epic conflict between Reason and Fear that bears on all mankind.