Harriet Welsch is a freelance agent.
The above is in keeping with my normal practice of identifying in the opening sentence the type of occupation, spy-fi wise, the individual best falls into. Since Welsch is not employed by any organization or company and does what she does because it interests her and she has her own long-term objective, freelance agent seemed most appropriate.
In truth, Welsch is an 11-year old girl attending elementary school while living with her parents in a nice home in the Upper East Side of New York City. That is a pretty high-end area of Manhattan largely consisting of upscale brownstones and shows right away that Welsch's parents were quite well off financially. This means that young Harriet is not in need of anything materially; emotionally is a different matter since her parents are largely absent most of the time off doing their own things. Rearing little Harriet falls then most to her nanny Catherine, whom she thinks of as 'Ole Golly'.
Harriet becomes a "spy" because she has a desire to grow up to be a writer and Ole Golly tells her to start practicing. To Harriet this means jotting down notes and observations in a notebook she will habitually carry. To give her something interesting to put in that notebook, she begins to walk a "spy route" each afternoon, observing her classmates, her family, and the people in the neighborhood. Who they were, what they did, how they seemed to interact - these were the tidbits that started to fill the journal as the more she saw, the more she became intrigued by little things and worked hard to learn the whys of the whats.