An orphaned boy living by his wits on the streets, who aids Sherlock in uncovering the truth behind a mysterious black smoke with his street-savvy resourcefulness.

An experienced American detective and tutor to Sherlock, who imparts critical sleuthing skills to the young boy while assisting in their investigation.

Sherlock's older brother who works in the Foreign Office, quietly influencing the outcome of events through his knowledge and connections.

An embittered French aristocrat intent on undermining British influence, whose schemes present a formidable challenge for Sherlock and his companions.

The assertive and brave daughter of Amyus Crowe, who actively participates in solving the mystery and demonstrates her own capabilities during their trials.

The housekeeper at Holmes Manor with a sour disposition towards Sherlock, whose actions suggest a link to wider conspiracies.

Sherlock's uncle, who is absorbed in his religious studies and remains largely unaware of the events unfolding around him.

A large and pragmatic enforcer for Baron Maupertuis, known for his readiness to follow orders without hesitation.