Petra is an 11-year-old middle school student in Ms. Hussey’s class. Her parents named her after an ancient stone city in Jordan—a mysterious place of buried secrets. Petra is quiet, introspective, and loves to write. Calder initially thinks she’s strange.

Petra dreams about the figure in A Lady Writing long before she knows the painting exists. She believes the Lady speaks to her and treats the figure like a human being whom she is determined to save. Petra is brave enough to trust her instincts in most instances, but sometimes doubts herself, as is the case when she believes she sees a pattern in the wrought iron that mirror Mrs. Sharpe’s pentomino words but doesn’t follow the clue. She is more adventurous than Calder, as she secretly follows her teacher before there is even a mystery to solve and even sneaks into a restricted area in the art gallery because she suspects the pieces back there are better.

Calder is an 11-year-old middle school student in Petra’s class who lives three houses away from her on Harper Avenue. Calder is interested in numbers, letters, and puzzles. He always carries a set of pentominoes in his pocket and believes they can offer him guidance in difficult situations.