Cass is an eleven-year-old girl who considers herself a strict survivalist. She is hypervigilant, constantly preparing for catastrophes that never occur, and always carries a heavy backpack full of emergency gear. Her eccentricity and catastrophic predictions isolate her from her peers until she teams up with Max-Ernest to investigate a mysterious box of scents.

Max-Ernest is an eleven-year-old boy with a highly logical mind and an undiagnosed condition that compels him to talk incessantly. His parents, who couldn't agree on his first name, are divorced but perfectly divide their shared house to avoid interacting. Ostracized by peers for his constant chatter, he forms an investigative alliance with Cass to decode a secret message.

Ms. Mauvais is a strikingly beautiful, wealthy woman who always wears long, white gloves. She presents a flawless, ageless appearance and takes a sudden interest in acquiring a specific notebook from the magician's estate. In the magician's old journal entries, a dangerously enticing figure known as the Golden Lady shares similar unnerving traits.

Dr. L is a handsome, impeccably dressed man who accompanies Ms. Mauvais. He speaks with an elusive, unidentifiable accent and, like his companion, wears long gloves at all times. In the notebook left behind by the magician, tales are told of a long-lost twin brother named Luciano Bergamo, casting a mysterious shadow over Dr. L's aggressive pursuit of the magician's secrets.

A local magician who dies without next of kin, leaving behind a quirky house and a mysterious box known as the Symphony of Smells. His hidden notebook, which details his childhood in a circus and his synesthesia, launches Cass and Max-Ernest on a dangerous quest to uncover his life's work.

The intrusive, fourth-wall-breaking author of the story (using the pen name Pseudonymous Bosch). He frequently interrupts the narrative to caution the reader about the danger of the secret he is sharing, while admitting he is too weak-willed to keep the mystery to himself.

A boy at Cass and Max-Ernest's school who is considered odd by his peers but beloved by art teachers. He experiences synesthesia, meaning his senses blend together, which inspires his award-winning abstract paintings.

Cass's supportive substitute grandfathers who live in a converted old fire station. They run an antique store downstairs and serve as stabilizing, understanding figures in Cass's life, even if they don't fully grasp her intense survivalist mindset.

A probate specialist who handles the sale of houses after their owners die, effectively acting as a real estate agent for the dead. She is a shallow, attention-seeking woman who views children condescendingly and inadvertently brings the Symphony of Smells into Cass's life.

Cass's mother, who worries deeply about her daughter's lack of friends and eccentric behavior. She leaves on a business trip early in the story, inadvertently granting Cass more freedom to pursue the mystery of the magician's death.

The exasperated school principal who frequently deals with Cass's catastrophic predictions. She tries to maintain order at the school and teach Cass not to cry wolf over minor incidents.

A popular classmate of Cass and Max-Ernest. She is obsessed with the Skelton Sisters celebrities and frequently gives away branded lip glosses to Cass, mostly out of pity.

A personal butler at the exclusive Midnight Sun spa who speaks with a heavy stutter. He is assigned to attend to guests and guide them to their specialized treatments.

An old, blind basset hound who serves as a loyal companion to Grandpa Larry and Grandpa Wayne at the converted fire station.