Who is the protagonist and who is the antagonist in "The Open Window"?
The protagonist of the story "The Open Window" is Framton Nuttel. Framton is a stranger to the Sappleton family, which he goes to meet for the first time at their home in the country, in order to stay with them. He goes there in hopes to rest to get a cure for his nervous condition. His sister, who had visited the area and met the family years ago, had sent with him letters of introduction so that the Sappletons know who Framton is.
Since Nuttel needs to cure his current nerve condition, anything or anyone that attempts against that, or against Framton, would be considered his antagonist. The definition of "antagonist" is
1. a person who is opposed to, struggles against, or competes with another; opponent; adversary.2. the adversary of the hero or protagonist of a drama or other literary work.
In this case, Nuttel's adversary, or antagonist, would be Mrs. Sappleton's niece, Vera. Vera is 15 years-old, described as "self-possessed", and the teller of the fictitious story about the deadly accident of Mrs. Sappleton's husband and her two young brothers.
Vera is his antagonist because she acted against Framton in full knowledge that her story would actually make his nerves even more delicate. Even though she was just being a mischievous teenager, she would still be considered his antagonist for this very reason.
Contrary to the antagonist, the protagonist of a story is the leading character. Framton is the leading character of "The Open Window" because he is at the center of the story, and because his main problem is also a big part of the plot.