Does the story have a protagonist? Does the story have an antagonist? If so, who?
In "The Monkey's Paw," the protagonist (or the leading character) is Mr. White. He is the protagonist because it is his friend, Sergeant-Major Morris, who brings the monkey's paw into the family home. Moreover, it is Mr. White who makes the first wish, setting in motion the plot's chain of events.
The antagonist, the person who opposes the protagonist, could arguably be the sergeant-major because he gives the monkey's paw to the family even though he knows that it possesses a very dark form of magic.
However, it could also be argued that Mrs. White plays the role of antagonist because, at the end of the story, she uses the monkey's paw to bring back her son, Herbert, even though Mr. White tells her that it is a "foolish" and "wicked" thing to do.
Through Mrs. White's actions at the end of the story, Mr. White's status as the protagonist is confirmed. He uses his third wish to wish Herbert away, thereby overcoming the evil powers of the monkey's paw.