Claire is a 12-year-old girl who loves science and actively dislikes anything related to the supernatural. She relies on logic and counting to manage her anxiety. Her rational worldview is challenged when she is forced to help run her father's ghost tour and begins experiencing unexplained phenomena.

Casley is Claire's long-time best friend who shares her passion for science and plans for the upcoming science fair. She is currently exploring new interests, including fashion and cosmetics. She maintains a forgiving and patient attitude, attempting to keep both her old and new friendships strong.

Emily is a new student in Chicago who recently moved in with her aunt. She enjoys wearing makeup and nice clothes, presenting a put-together exterior that intimidates some of her classmates. She uses her interest in fashion as a positive distraction from recent family difficulties.

Sam is Claire's older brother who plays baseball and attends her middle school. Though he enjoys playfully scaring his sister, he offers genuine support when she needs an ally. He struggles with his math classes and agrees to trade his investigative help for tutoring assistance.

Willie is a young boy from a family of Czech immigrants who died in a historical Chicago tragedy. Identified in historical records by a number, he represents the victims who are often forgotten by the public over time. Claire and her brother learn about his circumstances as they investigate local history.

Julia and David are small business owners in Chicago. Julia runs an online baking enterprise and looks for educational moments, while David operates a local ghost tour and writes books about paranormal history. David's intense professional focus on the supernatural causes friction with his highly rational daughter.

Warner is a middle school student with ruffled brown hair and gray eyes. He is observant and polite, showing a consistent interest in speaking with Claire. He partners with her on an academic project focusing on an early Chicago settler.