Who is the main character in "The Monkeys Paw?"

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:

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"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,

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"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.