In "The Open Window," the character who generates the effect is the niece, who oddly is not named. The second character is the one on whom the guest waits: the aunt, Mrs. Stappleton. Mrs. Stappleton is essential to creating the effect on Framton intended by the niece's "Romance at short notice." Thirdly the focal point of the irony and the victim of the ploy of the niece is Framton Nuttel, the guest.
It is interesting to note that while Framton's tone remains polite throughout the narrative, the tone of the niece as well as the tone of Mrs. Stappleton is somewhat mockingly patronizing:
'A most extraordinary man, a Mr. Nuttel,' said Mrs. Stappleton, 'could only talk about his illnesses, and dashed off without a word of goodbye or apology when you arrived...'
Then, when the niece fabricates another "explanation," the reader must wonder if Mrs. Stappleton is not really aware of the niece's penchant for "Romance at short notice."
(Perhaps Framton is not the first guest on whom such a trick has been played. If so, he is, then, the least important character although the reader tends to find him of greater importance on a first read.)
The three central characters in the story are fifteen-year-old Vera, the nervous Framton Nuttel, and Vera's aunt Mrs. Sappleton. Saki has taken care to create contrasting characters, which makes it easy for the reader to visualize them and to imagine the sounds of their voices. Vera and Framton are contrasting characters because she is young and female while he is middle-aged and male. Vera and her aunt belong to different generations. They do not interact with each other but only with Framton. Vera seems serious and "self-possessed," whereas Framton is a nervous wreck and Mrs. Sappleton strikes him as being slightly insane owing to the "family tragedy" fabricated by her precocious niece. Vera is by far the most original and interesting character in the story.