The author gives Vera's age in the opening lines of the story.
"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."
Saki wanted to write a story in which someone sets a visitor up to be terrified when three ordinary men arrive home and the visitor takes them to be ghosts. He may have thought of various types of characters who could tell the visitor such a story, but his choice of a fifteen-year-old seems perfect. She is just young enough to want to pull such a prank, but just old enough to be taken seriously. She is also old enough to be acting as a substitute hostess while her aunt is touching up her hair in front of the mirror and getting ready to come down.
Vera is bored. She wouldn't mind stirring up a little excitement. She is confined to the house because she is a girl. She can't go hunting like the men and probably wouldn't want to do it anyway because it involves getting muddy and killing innocent birds. She resents being used as a temporary hostess. She knows her aunt is just giving her some practice for the role she is destined to fill in a few more years when she gets married. She doesn't want to become an airhead like her aunt. She takes out her resentment on poor, gullible Framton Nuttel.
Vera is actually the most interesting character in the story. We may feel a little disapproving of the uproar she orchestrates, but we can't help liking her and admiring her for her wit and spunk. Saki twice describes her as self-possessed; and her adroit way of telling her preposterous story about the hunters being sucked into a bog underscores her coolness and self-possession. Her poise and disingenuousness are meant to contrast with her behavior when she pretends to see three ghosts approaching the open window.
Framton shivered slightly and turned towards the niece with a look intended to convey sympathetic comprehension. The child was staring out through the open window with a dazed horror in her eyes.
Saki calls her a child here. Vera is in that halfway zone between childhood and adulthood. The child in her is loving this game she is playing with this skittish visitor. We can imagine the goggle-eyed look she is faking for his benefit. It is enough to make him run for his life.
Framton grabbed wildly at his stick and hat; the hall door, the gravel drive, and the front gate were dimly noted stages in his headlong retreat. A cyclist coming along the road had to run into the hedge to avoid imminent collision.