Claudia Kincaid
A twelve-year-old girl, Claudia is an intelligent, spirited child who loves nice things. Frustrated at what she perceives as her poor treatment by her family, she resolves to run away to the Metropolitan Museum of Art, which suits her extravagant tastes. She is a born leader, though she can be a bit dramatic.
Jamie Kincaid
Claudia's younger brother. He is very careful with money and amassed a great deal of savings. It is for this reason that Claudia decides to bring him along on her trip to the Met. Jamie is no less intelligent than his sister, though he is somewhat more level-headed, and has a dry but strong sense of humor.
Mrs. Basil E. Frankweiler
As the narrator of the story, Mrs. Frankweiler is the one who tells us about the adventures of Jamie and Claudia. A wealthy 82-year-old widow, she amassed a great fortune, which includes a mysterious art object that may have been carved by the great Michelangelo. Though Mrs. Frankweiler seems standoffish and rude at first, this prickly personality hides a gentle and caring heart.
Saxonberg
Much of the book - in the form of Mrs. Frankweiler's letters - is addressed to someone named Saxonberg. He is ultimately revealed to be her lawyer, as well as the grandfather of Claudia and Jamie. The slow reveal of this fact is yet another mystery at the heart of the book. Mrs. Frankweiler considers him a very important person in her life, almost like a family member, though she often speaks roughly to him.
The Kincaid Family
The parents of Claudia and Jamie, as well as their brothers and sisters, are not characters we see often in the book. Their parents have a total of four children, are very busy, and the kids themselves are not very close to each other. Their youngest brother Kevin is described as a major burden.