Robert is the second son of Laird George Colwan and Rabina, though he is raised almost entirely by his mother and the preacher Robert Wringhim. Instructed as a staunch Calvinist, he believes he is a justified person who is predestined for heaven, which gives him a sense of spiritual superiority. He is highly intelligent but deeply antisocial, harboring intense resentment toward the father and brother who live without him.

Gil-Martin is a mysterious, highly persuasive figure who quickly befriends Robert Wringhim Colwan. He possesses an uncanny ability to change his appearance and adopt the likenesses and mannerisms of other people. Well-versed in theology, he uses his intelligence and strange abilities to validate Robert's most extreme beliefs and encourage him toward radical actions.

George is the eldest son of Laird George Colwan and the legal heir to the Dalcastle estate. Unlike his estranged younger brother, George is charming, well-adjusted, and enjoys a secular life filled with friends, sports, and socializing. He is generally generous and forgiving, often attempting to make peace with the brother who continuously harasses him.

The unnamed Editor is the narrator of the novel's framing story, living more than a century after the events take place. Operating as a skeptical, rational investigator, the Editor attempts to piece together the history of the Colwan family from old documents, court records, and witness accounts. The Editor maintains a detached, objective tone while examining the religious fanaticism of the past.

Laird Colwan is a Scottish nobleman and a Member of Parliament who enjoys drinking, dancing, and secular entertainment. His extravagant lifestyle deeply offends his strict, religious wife, leading to the rapid breakdown of their marriage. He dotes entirely on his eldest son, George, while refusing to acknowledge his second son, Robert.

Rabina is the strict, devoutly Calvinist wife of Laird Colwan. Appalled by her husband's worldly behavior, she quickly separates herself from him, preferring a life of religious study and seclusion. She relies heavily on her priest, Robert Wringhim, and raises her second son with the same severe religious principles she follows.

Robert Wringhim is a strict Calvinist preacher and a prominent figure in the local Whig political faction. He serves as Rabina's primary spiritual advisor and takes on the role of surrogate father to her second son, who is named after him. He teaches a doctrine of predestination, assuring his followers that they are inherently justified and immune to damnation.

Arabella Logan is Laird Colwan's mistress, acting as the primary female figure of the Dalcastle estate. She helps raise young George and develops a strong, protective bond with him. Resourceful and fiercely loyal to the Colwan family, she takes it upon herself to investigate mysterious circumstances surrounding violent conflicts in the city.

Bell Calvert is a sex worker living in poverty in Edinburgh. She becomes a key witness to violent events in the city streets, leading her to form an investigative alliance with Arabella Logan to uncover the truth.

Drummond is a social acquaintance of George Colwan who becomes entangled in the city's growing violence. After a public disagreement with George, he finds himself falsely accused of crimes, complicated by the existence of a man who looks exactly like him roaming the streets.

James Hogg is an author and local resident whom the Editor contacts for information regarding local folklore and a mysteriously preserved body. He is hesitant to discuss the grim local legends, prompting the Editor to investigate further alone.

Adam Gordon is a close, supportive friend of George Colwan. He attempts to provide rational explanations for the bizarre stalking George endures, offering sympathy when George's own father struggles to understand the situation.

Baillie Orde is a resident of Glasgow and Rabina's father. Valuing marital duty over his daughter's comfort, he refuses to grant her refuge from Laird Colwan, enforcing her return to her unhappy marriage.

Linton is a somewhat unstable man who works for James Watson's publishing company. He provides Robert with a place to stay when Robert arrives in the city seeking anonymity.

James Watson is the owner of an Edinburgh publishing company. He briefly employs Robert and agrees to print his religious pamphlet before becoming disturbed by its contents.