The Kincaids live in Greenwich, Connecticut, a suburb of New York City where Mr. Kincaid works. Mrs. Frankweiler lives on a "country estate" in Farmington, Connecticut, closer to Hartford.
- Claudia Kincaid, 12, is the oldest of four children and the only girl, so she both sets the table and empties the dishwasher. She is a straight-A sixth grade student, a critic of English grammar, and a good planner, except about money which she spends largely on sweets. She feels unappreciated at home, plans to run away with her brother Jamie, and recruits him. They run to the Metropolitan Museum and there discover a mystery of the art world, which fascinates her and overwhelms the adventure.
- Jamie Kincaid, 9, is the third child and the middle boy, in fourth grade, quiet and frugal. He complements his sister perfectly: "adventurous (about everything but money) and rich" —from their viewpoint, an American suburb in the mid‑1960s. He cheats at the card game War, playing with his best friend for money on the school bus daily. From that and his weekly allowance he has saved $24.43, and he has a transistor radio, his one purchase.
- Mrs. Basil E. Frankweiler, 82, is smart, insightful, eccentric, and rich. She is the narrator, telling the story of Claudia and Jamie Kincaid to her attorney. She is a commentator, providing insight into the children's actions. She is plot facilitator, for her election to allow sale of an extraordinary sculpture at auction, for only $225, set the book's mystery in motion.
- Saxonberg is Mrs. Frankweiler's lawyer, and is revealed to be Claudia and Jamie's grandfather.