While the obvious assumption is that Mr. White is the main character, an argument can be made that Herbert White is, indeed, the principal character since he prompts his father to make a wish upon the monkey's paw, and he is the character around whom the plot centers.
When Sergeant Major Morris visits his old friend, Mr. White, he pulls from his pocket a monkey's paw, given to him by an old holy man, a fakir. The older White asks him why he keeps it; then, the sergeant throws the paw into the fire. Seeing the paw discarded so, Mr. White is tempted by it and retrieves it from the fire; however, it is Herbert who, in his frivolity, actually encourages his father to make a wish:
"Likely," said Herbert, with pretended horror. "Why, we're going to be rich, and famous, and happy. Wish to be an emperor, Father, to begin with: then you can't be bossed around."
As he runs around the table, Mr. White takes out the paw from his pock and "eyed it dubiously." This action indicates that he has had no intention of using it--at least immediately. Herbert encourages him,
"If you only cleared the house, you'd be quite happy, wouldn't you?"..."Well, wish for two hundred pounds, then: that'll just do it."
So, even though Mr. White makes the fateful wishes, he is prompted to do so by his son Herbert. And, in the other two cases, the dire states of Herbert are what effect the wishes made by Mr. White and drive the plot of W. W. Jacobs's story.
The main character in "The Monkey's Paw" is Mr. White, Herbert's father. Though Herbert, Mrs. White, and the Sergeant-Major all play central roles in the story, Mr. White is the one who expresses such fascination for the Indian culture, who saves the paw from the fire, and who makes the first wish—a wish that costs the couple the life of their son, Herbert.
What elevates Mr. White's importance to driving the plot to its conclusion, is Mrs. White's insistence that her husband wish back their dead son.
Mr. White does so at his wife's urgent pleadings, but the cold reality of what he has asked for, and what might appear at their door, drives Mr. White to utter his final wish, that his son remain dead.
As the plot centers around Mr. White and his actions, which galvanize the plot forward to its conclusion, I would choose Mr. White as the main character.
The main character in this spine-chilling short story is without a doubt Mr. White senior, the father of the family who we are presented with on the "cold and wet" night when Sergeant Major Morris enters their lives with the story of the monkey's paw that comes to dominate the rest of the narrative. Interestingly, the first paragraph shows Mr. White's risky nature and his love of experimentation, which perhaps reflects why he is so eager to try out the monkey's paw with a wish:
Father and son were at chess, the former, who possessed ideas about the game involving radical changes, putting his king into such sharp and unnecessary perils that it even provoked comment from the white-haired old lady knitting placidly by the fire.
Mr. White, then, is established as a risk taker in the very first paragraph in the story - something that foreshadows his trial of the monkey's paw and the tragic consequences. The rest of the story on the whole focuses on him and his gradual understanding of the power of the monkey's paw and how interfering with the fate that rules our lives only brings us sorrow - as the fakir created the monkey's paw to show us.