Saki invented Vera to suit the needs of his plot. He wanted to have a character who would tell the neurotic visitor a wild story about three men being sucked into a bog and her aunt leaving the French window open for three years because she expected them to come home from their hunting excursion. Saki could have used Mrs. Sappleton herself for this purpose. He could have invented some eccentric family servant or a demented old family member who escaped from confinement in an upstairs room and slipped into the living room to tell Framton the story while the nervous visitor was waiting for Mrs. Sappleton to arrive. But Saki's choice of a precocious and mischievous fifteen-year-old girl was the best. Vera had to be old enough to be convincing but young enough to take a sadistic delight in scaring their guest with a ghost story.
Saki emphasizes the two aspects of Vera's character by the descriptive terms he uses.
"My aunt will be down presently, Mr. Nuttel," said a very self-possessed young lady of fifteen; "in the meantime you must try and put up with me."
Here she is introduced as a young lady. Who would suspect this self-possessed young lady of making up such a horrible story? She not only fools Framton Nuttel, but she fools us readers completely.
But when Mrs. Sappleton cries, "Here they are at last!" and Framton turns to look at Vera with "sympathetic comprehension":
The child was staring out through the open window with dazed horror in her eyes.
Vera is neither a child nor an quite an adult: she is an adolescent. As such, she is practicing being an adult but still capable of doing childish things, such as frightening a nervous visitor who is a complete stranger half to death. If Saki had made her a bit older, she would have been less likely to think of such a zany stunt; and if her creator had made her a bit younger, neither Framton nor the reader might have been so thoroughly taken in by her ghost story and by the girl's imitation of dazed horror. Necessity is said to be the mother of invention. By inventing a character to suit a very specialized need, Saki created one who is realistic, memorable, amusing, and engaging.
The character of Vera is conveyed through the way in which she sizes up Framton Nuttel very carefully, checking his knowledge of her family, before launching into her tale and tricking him. It is clear from this that she is very intelligent and also incredibly bright. Note how this is shown through the following question, that is repeated in a different form two times in order to ascertain Framton's precise level of knowledge of the family:
"Then you know practically nothing about my aunt?" pursued the self-possessed young lady.
The description of Vera as "self-possessed" clearly indicates the kind of young lady that Vera actually is. The ease with which she manages to trick Framton and to play a joke on him, that she does not even share with others suggests very strongly that she is very much in control of the situation around her and is able to trick and manipulate both strangers and her own family with ease in a very creative and flexible manner. She is certainly presented as a force to be reckoned with, as Framton finds out to his cost.