Tessa is a sixteen-year-old orphaned American who travels to London expecting to reunite with her brother, Nate. Instead, she is intercepted by the Dark Sisters and discovers she possesses the supernatural ability to shape-shift by holding objects belonging to others. She is an avid reader who must quickly adapt to the hidden reality of Shadowhunters, warlocks, and demons while trying to locate her missing brother.

Will is a seventeen-year-old Shadowhunter residing at the London Institute. He is highly capable in combat but masks his true emotions behind a facade of sarcasm, unpredictable behavior, and deliberate emotional distance. He discovers Tessa's imprisonment while investigating a mundane murder and brings her into the safety of the Shadowhunter world.

Jem is a seventeen-year-old biracial Shadowhunter who relocated from Shanghai to London following the death of his parents. He is calm, rational, and openly empathetic. He provides a stabilizing presence at the Institute and acts as a counterbalance to Will's erratic behavior, though he privately manages a severe, chronic supernatural illness.

Charlotte is the twenty-three-year-old Head of the London Institute. She is a dedicated leader who frequently faces scrutiny from the older, male members of the Clave due to her gender and progressive approach to leadership. She offers Tessa sanctuary and commits the Institute's resources to finding Nate.

Mrs. Dark is one of the two enigmatic women who meet Tessa at the Southampton docks under the guise of helping Nate. She runs the Dark House, where she uses cruel methods to force Tessa to develop her shape-shifting abilities for an unnamed master.

Mrs. Black is the sister of Mrs. Dark and co-conspirator in Tessa's kidnapping. She assists in keeping Tessa imprisoned and oversees her rigorous, painful training sessions with objects belonging to the deceased.

Alexei de Quincey is the prominent head of a London vampire clan. He operates within high society and allegedly runs the Pandemonium Club, an establishment that mixes mundane and supernatural patrons, drawing the scrutiny of the Shadowhunters.

Nate is Tessa's older brother who moved to London for work following the death of their Aunt Harriet. He is Tessa's only living relative and primary source of financial support. His sudden disappearance upon Tessa's arrival in England draws her into the hidden supernatural society of London.

Axel Mortmain is Nate Gray's mundane employer in London. He is a businessman who holds membership in the Pandemonium Club, an establishment that introduces him to the supernatural elements of the city. He provides the Shadowhunters with early leads regarding Nate's connections.

Henry is a Shadowhunter and Charlotte's husband. He spends much of his time in the Institute's crypt laboratory, creating elaborate inventions and weapons that frequently malfunction. He assists in Tessa's rescue and helps his wife manage the Institute's administrative duties.

Jessamine is a teenage Nephilim orphan living at the London Institute under Charlotte's care. Unlike the others, she actively rejects her Shadowhunter heritage and combat training. She focuses entirely on fashion and her desire to enter normal Victorian high society.

Sophie is a human servant employed at the London Institute. She possesses the "Sight," allowing her to see through supernatural glamours. She assists Tessa in acclimating to life at the Institute and holds strong opinions about the resident Shadowhunters.

Camille is an elegant vampire who approaches the London Institute as an informant. Seeking retribution against her former partner, she offers to help the Shadowhunters gather evidence of treaty violations by allowing Tessa to shape-shift into her form to infiltrate a vampire gathering.

Magnus Bane is a warlock and Camille Belcourt's current romantic companion. He attends high-society Downworlder events and possesses extensive knowledge of spellwork, assisting the Shadowhunters during their infiltration of a vampire gathering.

Thomas is a human servant who works at the London Institute alongside Agatha and Sophie. He assists the Shadowhunters with physical labor, preparations for missions, and managing the property's defense.

Agatha is the cook at the London Institute. As a mundane with the Sight, she provides domestic support for the resident Shadowhunters and handles the daily preparation of meals.

Brother Enoch is a member of the Silent Brothers, an order of Shadowhunters who serve as healers and scholars. He communicates telepathically and attends to the medical needs of those at the London Institute.

Miranda is a clockwork automaton covered in human flesh that acts as a servant at the Dark House. She tends to Tessa during her captivity before being disabled and transported to the Institute for study.

Emma Bayliss is a young murder victim discovered by Will and Jem in a London alleyway. Her death initiates the Shadowhunters' investigation, and her personal ribbon is later used by the Dark Sisters to test Tessa's shape-shifting abilities.

Benedict Lightwood is a senior, influential member of the Clave. He holds traditional views regarding Shadowhunter law and frequently challenges Charlotte Branwell's authority and capability as the Head of the London Institute.

Aunt Harriet is Tessa and Nate's deceased guardian. She raised the siblings in New York City after the death of their parents, taking on the burden of managing their household and finances until her passing.