Petra is an 11-year-old, sixth-grade student at the University of Chicago's Laboratory School. As the oldest of five children in a loud household, she frequently seeks quiet spaces to read and think. She routinely visits Powell's Bookstore to hide among the shelves and observe the patrons. She relies on her intuition and books, such as Charles Fort's *Lo!*, to make sense of her surroundings.

Calder is an 11-year-old student in Ms. Hussey's class who lives on the same street as Petra. He operates in a quiet household as an only child. He carries a set of mathematical pentominoes at all times, using the geometric shapes to spell words, solve problems, and communicate in code. He works a part-time job at the local bookstore.

Mrs. Sharpe is an imposing, elderly resident of Hyde Park with striking green eyes. She is the widow of a murdered Vermeer scholar. She possesses a vast collection of books and art, including pieces directly connected to Johannes Vermeer. Her sharp intellect and direct conversational manner make her an intimidating figure.

Ms. Hussey is a teacher at the University of Chicago Laboratory School. She prefers unconventional assignments, such as asking her class to find everyday objects that feel like art or searching for physical letters in a modern age. She encourages her students to abandon traditional thinking and draw their own conclusions about their environment.

Xavier Glitts is an international criminal operating in the Hyde Park area. He specializes in high-stakes art theft and complex confidence schemes. He embeds himself into local communities to identify experts, gather resources for his operations, and manipulate innocent residents.

Mr. Watch is the middle-aged manager of Powell's Bookstore who consistently wears a pair of red suspenders. He employs Calder for part-time work and supplies reading material to local academics. He harbors a deep interest in mysteries and the art world, viewing his bookstore as a hub for both subjects.

Tommy is Calder's lifelong friend who left Hyde Park after his mother remarried. He maintains contact with Calder through a secret, alpha-numeric code based on pentomino pieces. He keeps his friend updated on unusual occurrences in his new neighborhood, expressing distrust of his changed living situation.

Frog is a young boy who lives next door to Tommy in New York. His sudden disappearance from his neighborhood prompts Tommy to investigate and send coded letters to Chicago asking for advice.

Grandma Ranjana is Calder's deceased grandmother who possessed a deep appreciation for the painter Johannes Vermeer. She bequeathed Calder a Geographer's box that sparks his interest in the artist's work and influences his investigation.