What we know about Mr. Framton Nuttel in "The Open Window" by Saki (H. M. Munro) is that he has no social graces, which is to say he doesn't quite know how to conduct himself in social situations, like that of paying a visit to total strangers: "Framton Nuttel endeavored to say the correct something." We also know that he makes a habit of staying by himself and not mixing with other people, as his sister says to him: “you will bury yourself down there and not speak to a living soul.” We know he has some sort of nervous complaint for which he is taking a "nerve cure" that entails "complete rest, an absence of mental excitement, and avoidance of anything in the nature of violent physical exercise." And we know that Framton is doubtful of the added benefit to his nerve cure that might be derived from meeting complete strangers to whom his sister has written "letters of introduction."
We also know that his limited idea of conversation, which “labored under [a] tolerably widespread delusion,” includes "only talk about his illnesses." We also know that Framton is impressionable and naive and gullible. We know all this for two reason: the way he describes the young niece, Vera, and the insight of hindsight we get at the close of the short story. The narrator often reflects Framton's own observations such as when describing the niece as "a very self-possessed young lady of fifteen"; as when later the narrator adds, "pursued the self-possessed young lady"; and later still says: "The child was staring out through the open window with a dazed horror in her eyes." The insight of hindsight that informs us of Framton's impressionable gullibility comes when Vera starts to make up an appalling story to explain Framton's behavior--just as she did to fill in the void of Mrs. Sappleton's absence (notice how Sappleton is reminiscent of “sap,” which means “a gullible or foolish person” (World English Dictionary)).
Speaking of whom, what we know of Mrs. Sappleton is that she is bright, cheerful, and energetic: "the aunt bustled into the room." We also know that she is very enthusiastic in her praise of the activities of the men she cares about, her husband and brothers (and their little spaniel dog). We also know she is big-hearted and gracious when bored by tedious visitors (like Framton Nuttel) but not at all unrealistic in her assessment of strangers:
A most extraordinary man, a Mr. Nuttel, ... dashed off without a word of goodby or apology.... One would think he had seen a ghost.
We also know that she is good natured and a doting wife and loving sister, even when teased: "I said, Bertie, why do you bound?" Finally, we know that Mrs. Sappleton doesn't understand Vera in the least. She never suspects for a moment in the story that Vera is wreaking havoc with people's imaginations through wild stories she convincingly concocts in an instant, like the one about the men being lost in a quicksand bog and the one about "’a cemetery somewhere on the banks of the Ganges [with] a pack of pariah dogs.’ … Romance [adventure] at short notice was her speciality."