Quentin is a high school senior living in a subdivision in Orlando, Florida. Raised by two professional therapists, he is generally well-adjusted but strongly prefers routine, boredom, and strict adherence to rules. He harbors a lifelong infatuation with his next-door neighbor, though their friendship faded years ago. His preference for the sidelines is challenged when he is pulled into an unexpected midnight adventure.

Margo is Quentin's next-door neighbor and an incredibly popular senior at Winter Park High School. She is famous for her extravagant adventures, mysterious schemes, and fiercely independent attitude. Despite her high social status, she views her Florida hometown as an artificial environment filled with fake people. She often runs away from home, leaving cryptic clues behind for others to find.

Ben is one of Quentin's best friends and a fellow band member at Winter Park High School. He provides constant energy and humor to their friend group, though his primary focus is usually on climbing the social ladder. He struggles to overcome a vicious rumor spread by a popular girl during his sophomore year, which severely damaged his reputation.

Radar (real name Marcus) is the highly intelligent and perceptive third member of Quentin's core friend group. He spends most of his free time obsessively updating and moderating an online reference site called Omnictionary. He frequently acts as the mediator when Quentin and Ben argue about social priorities.

Lacey is a popular senior at Winter Park High School and Margo's best friend since kindergarten. She exists at the top of the high school social hierarchy but struggles with the pressure to maintain her status. She finds herself caught in the crossfire of her friends' messy romantic entanglements.

Jase is Margo's boyfriend and a wealthy baseball player at the high school. He lives in an opulent subdivision equipped with heavy security and occupies the highest tier of popularity among the students. His unfaithfulness serves as the catalyst for Margo's destructive rampage through Orlando.

Becca is a popular, attractive student who previously held a close friendship with Margo. She is known for her cruel streak, having started a vicious rumor about Ben that damaged his high school experience. She becomes the primary target of Margo's midnight revenge.

Chuck is a large, physically imposing athlete who acts as the school's primary bully. He enjoys tormenting underclassmen and less popular students, using his physical size and social status to intimidate others with impunity.

Angela is Radar's new girlfriend. She is friendly and communicative, though she grows increasingly concerned about why Radar refuses to invite her to his home after five weeks of dating. She is unaware of his parents' unusual collections.

Mrs. Jacobsen is Quentin's mother. As a professional therapist, she frequently analyzes her son's behavior, attempting to ensure he remains well-adjusted during his high school years. She approaches parenting with a calm, clinical demeanor.

Mr. Jacobsen is Quentin's father and also a practicing therapist. He often collaborates with his wife to analyze social behaviors, attempting to maintain a structured household while encouraging Quentin to process his emotions.

Mrs. Spiegelman is Margo's mother. She is highly concerned with her family's public image in their subdivision and finds her eldest daughter's rebellious nature exhausting. She frequently reaches her breaking point regarding Margo's stunts.

Mr. Spiegelman is Margo's father. He attempts to enforce strict boundaries on his daughter, though his primary concern often centers on what the neighbors think of his family's domestic issues.

Ruthie is Margo's younger sister. She lives in Margo's shadow but is highly observant of the household dynamics. She is entirely willing to bypass her parents' strict rules if offered a small bribe.

Detective Warren is the police officer assigned to handle Margo's case. Having dealt with runaway teenagers extensively, he approaches the situation with a weary but realistic perspective, treating Margo as a typical rebellious youth rather than a profound mystery.

Robert is a local man whose tragic suicide Quentin and Margo witness when they are nine years old. The trauma of finding him deeply impacts both children, leaving a lasting impression on Margo regarding the fragility of human connection.

Dr. Holden is an English teacher at Winter Park High School. She holds a deep appreciation for literature and attempts to guide her students toward constructive interpretations of complex poetry, acting as a sounding board for Quentin.

Gus is a Winter Park High School alumnus who now works night shifts as a security guard. He is friendly with Margo and allows her special access to the building he guards, reflecting her wide network of unconventional acquaintances.

Myrna Mountweazel is the Spiegelman family's pet dog. Margo frequently factors the dog's predictable behavior into her complicated plans for sneaking in and out of the house unnoticed.