Emily is a seventh-grader who recently moved to San Francisco. She is a passionate reader and an avid player of the online book-hunting game Book Scavenger. Having lived a transient lifestyle due to her parents' adventurous nature, she deeply desires to stay in San Francisco permanently and seeks a part-time job to help her parents with finances. She struggles with public speaking and self-doubt but pushes herself to join the school dance committee to become more involved.

James is a cheerful, energetic seventh-grader who lives in the apartment above Emily. His parents own the building and run a catering business. He shares Emily's love for codes and the Book Scavenger game, frequently communicating with her via a secret pulley system rigged between their windows. He wants to find the rumored unbreakable code's treasure to convince his father to stop traveling for work.

Mr. Quisling is Emily and James's social studies teacher. He is a longtime fan of Book Scavenger, originally joining the game to maintain a connection with his son following a divorce. He holds a deep, personal interest in a local historical mystery known as the unbreakable code, having worked on an excavation of a buried Gold Rush ship years earlier.

Mr. Sloan is an aspiring substitute teacher who visits Emily's school. He attends the Book Scavenger event at Hollister's and tries to network with Mr. Quisling for teaching opportunities, occasionally offering unsolicited advice about classroom management. He steps in to cover classes when the regular teacher goes on bed rest.

Matthew is Emily's older brother. He possesses a natural charisma and social ease that his sister envies. With his bold clothing choices and love for music, he provides levity and steady support to Emily and James during their puzzle-solving excursions, often joining them when they need backup.

Hollister is the kind, dreadlocked owner of a local independent bookstore that serves as a community hub for Book Scavenger players. He offers Emily a part-time job to help ease her family's financial strain. He remains a generous and resilient figure in the neighborhood despite facing severe setbacks to his business.

Mr. Griswold is the eccentric founder of the Book Scavenger game and the head of Bayside Press. Following a recent physical attack, he suffers from trauma, becoming reclusive and relying heavily on his service dogs and office security cameras. He forms a teen advisory committee to help guide the game's future while he works to recover his confidence.

Charlie is a college student hired to build a website for Hollister's bookstore. He has an aloof demeanor and wears large gauges in his ears. He holds a bitter grudge against his former middle school teacher over a bad grade, and he frequently shirks his responsibilities at the bookstore when unsupervised.

Emily's parents are freelance workers with a spontaneous and adventurous approach to life. They have moved their family across the country multiple times in search of new experiences. Facing financial difficulties due to a bankrupt client, they privately consider moving away from San Francisco, prompting Emily to look for ways to help them stay.

Ms. Linden is a librarian at the History Center. She has colorful hair, tattoos, and a deep knowledge of San Francisco history. She acts as an unconventional adult ally to the kids, sharing historical documents related to the Gold Rush and offering her boat to help them explore the bay.

Jack is Garrison Griswold's assistant at Bayside Press. He helps manage the teen advisory committee and acts as a protective buffer for his boss, managing events and communications while Mr. Griswold recovers from his injuries.

Miranda is a woman from Mr. Quisling's past who participated alongside him in an archaeological dig. Emily and James attempt to contact her online, believing she might hold answers to the puzzles they are trying to solve.

Maddie is a seventh-grade classmate of Emily and James. She sports a distinct haircut and harbors a long-standing, unexplained grudge against Emily. Despite her prickly demeanor, she participates in the school dance committee and helps organize events.

Vivian is the seventh-grade class president. She initially attempts to control the school dance planning by forcing a strict theme, but she eventually learns to incorporate ideas from the rest of the committee.

Mrs. Ortega is Emily and James's pregnant teacher. After experiencing early contractions during a class period, she is placed on bed rest, requiring a substitute to take over her classroom duties.

James's father runs a Chinese catering business and acts as the landlord for Emily's family. His job requires frequent travel, creating emotional distance between him and his son, which James hopes to bridge by finding a valuable treasure.

James's grandmother is an older relative who assists the children in their puzzle-solving. She provides crucial linguistic help by translating hidden Chinese characters discovered on an old historical document.

Devin is a seventh-grade student and twin who participates in the school dance committee. He and his brother bring enthusiasm and silly costumes to the event planning, though they sometimes spread unverified neighborhood rumors.

Kevin is a seventh-grader and Devin's twin brother. He participates in planning the school dance and enjoys the chaotic fun of the events, though he accidentally damages a classmate's poster during a committee meeting.

Nisha is a classmate of Emily and James. She attends the Book Scavenger celebration at Hollister's and expresses admiration for their puzzle-solving skills, helping Emily feel more accepted by her peers.