The Kincaids live in Greenwich. Mrs. Frankweiler lives on a "country estate" in Farmington, Connecticut.
- 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.
- James "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"[5] —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, having a gross tradition for not shuffling their cards. He often bothers his sister about money along their journey. From that and his weekly allowance he has saved $24.43 (equal to about $199 in 2021), and he has a transistor radio, his one purchase. Jamie was recruited treasurer of the team because of the way he spends his money carefully.
- 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 the 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 as a huge surprise in the end of the book, and Claudia and Jamie have yet to find out.
