Richard is a mild-mannered businessman in his twenties living a deeply ordinary life in London Above. He tends to let life happen to him rather than taking charge, passively following the instructions of his ambitious fiancée. His predictable existence is permanently upended when he chooses to help a bleeding girl on the street, an act of compassion that plunges him into the dangerous magical realm of London Below.

Door is a young woman from London Below and the last surviving member of the aristocratic House of Arch. She possesses her family's unique magical ability to open and create passages where others cannot. Driven by survivor's guilt, she seeks to uncover the truth behind her family's sudden murder.

Mr. Croup is a highly intelligent and talkative assassin operating in London Below. Dressed in an off-putting suit, he serves as the brains of a deadly duo hired to hunt down Door. He delights in violence, destruction, and boasting of historical atrocities he claims to have caused.

Mr. Vandemar is a massive and exceedingly grim assassin who works alongside Mr. Croup. While he lacks his partner's extensive vocabulary, he shares a ruthless passion for violence. He possesses uncanny abilities, including the capacity to track targets by scent and a bizarre affinity for consuming oddities.

The Marquis is a mysterious trickster dressed entirely in black. He deals exclusively in the currency of favors and approaches most interactions as strict transactions. Though often perceived as self-serving and unpredictable, his deep knowledge of London Below makes him an invaluable guide.

Hunter is a highly skilled bodyguard in London Below, recognizable by her leather clothing. She has dedicated her life to slaying mythic beasts and takes immense pride in her legendary martial prowess. She joins Door's party to provide essential physical protection against the threats of the Underside.

Islington is an ancient angel residing in a subterranean citadel. Described as neither male nor female, the angel is a mythical being from whom Door seeks guidance regarding her family's murder. Islington tasks the group with retrieving a special key from the Black Friars.

Jessica is Richard's strong-willed fiancée. She met Richard at the Louvre and took it upon herself to mold him into a more successful version of himself. She is deeply concerned with appearances and social mobility, prioritizing her career over personal connections.

Anaesthesia is a young rat-worshipper who originally lived in London Above before leaving a bad home life to join the hidden society of London Below. She serves as an early guide for Richard, explaining the harsh rules of her world and bearing a quartz bead necklace earned at a previous market.

Old Bailey is a crotchety inhabitant of London Below who resides on the rooftops. He keeps company with birds and holds onto special items for his associates, operating as a vital informant and eccentric fixture of the hidden city.

Lord Portico is Door's murdered father and the former patriarch of the House of Arch. He was a visionary aristocrat in London Below who believed in uniting the divided factions of the Underside, a philosophy that left behind powerful enemies.

Garry is Richard's coworker in London Above. He is a perfectly ordinary office worker whose inability to recognize or remember Richard underscores the terrifying reality of Richard's exile from the normal world.

Serpentine is a prominent lady of London Below who owns a stable. She views Lord Portico's plans to unite the Underside as foolish but provides crucial food and sanctuary to Door's party out of respect for her former employee.

Lamia is a pale woman belonging to a group known as the Velvets, who sleep during the day and emerge at night. She offers her services as a guide through the dangerous depths of Down Street, though her assistance carries a dangerous supernatural price.

Lord Rat-speaker is a fur-covered man who presides over a community of rat-worshippers. He is capable of communicating directly with rats and directs his people based on the rodents' chittering instructions.

The Abbot is the leader of the Black Friars, an order of monks guarding a sacred artifact. He administers a deadly ordeal to anyone seeking the key they protect, maintaining a wall memorializing those who have failed the test.

Mr. Stockton is Jessica's wealthy and influential boss. He is a prominent figure in London Above, sponsoring high-profile museum exhibitions and commanding immense respect from his employees.

The Earl is a royal figure of London Below who holds court inside a traveling train carriage. He is an old friend of Lord Portico but harbors a fierce grudge against the Marquis, whom he previously banished.