Roderick is a seventeen-year-old tenant farmer living in the Scottish Highland village of Culduie during the mid-19th century. He stands accused of committing a brutal triple homicide that shocks his community. Though highly intelligent and observant, he cultivates a withdrawn persona that alienates him from his peers. He views his rural life with frustration and harbors deep resentment toward the rigid class system governing his family's existence.

Lachlan is a robust crofter in Culduie who nurses a long-standing feud against the Macrae family. After securing the position of parish constable, he leverages his newly acquired authority to wage a systematic campaign of harassment against his neighbors. He views his power as an opportunity to satisfy personal grievances under the guise of official duty.

John is Roderick and Jetta's father, a struggling crofter whose lot in life declines following the death of his wife in childbirth. He retreats into melancholy and emotional distance, interpreting all hardship as divine punishment for his sins. This fatalistic view makes him dangerously passive in the face of Lachlan Mackenzie's escalating abuse.

Jetta is Roderick's older sister and the emotional anchor of the Macrae household following their mother's death. Unlike her withdrawn brother and father, she is naturally extroverted and openly embraces local folk superstitions. She claims to receive prophetic visions of the future, which guide her fierce protective instincts toward Roderick.

Flora is Lachlan Mackenzie's teenage daughter, a practical young woman who dreams of moving away to Glasgow to find a husband. She readily accepts the social hierarchies of her community, putting her at odds with Roderick's rebellious worldview. Despite the bitter animosity between their fathers, she attempts to maintain polite interactions with Roderick until his behavior makes her uncomfortable.

Andrew is Roderick's legal counsel, tasked with defending him against murder charges in a highly publicized trial. He hopes to establish a defense based on "moral insanity," a nascent and widely misunderstood psychological concept. He commissions Roderick to write his memoirs to build this case, working to shape the narrative to save his client.

Thomson is an educated criminologist well-versed in the emerging field of criminal anthropology. He travels to Inverness to evaluate Roderick's mental state prior to the trial. He holds a high opinion of his own intellect and firmly believes that criminal behavior correlates with observable physical traits and genetics.

Archibald is a loquacious and worldly boy roughly the same age as Roderick. Having spent time around the gentry, he adopts an air of sophistication that easily impresses his more sheltered peer. He acts as a chaotic influence, encouraging Roderick to drink heavily and aggressively pursue romantic impulses.

The Factor is the appointed representative for Lord Middleton, charged with managing the estate's tenant farmers. He enforces the estate's unwritten rules with arbitrary cruelty, viewing the crofters as ungrateful inferiors. He casually dismisses John Macrae's complaints about local corruption.

Donnie is Lachlan Mackenzie's toddler-aged son. He lives in the Mackenzie home alongside his father, sister, and senile grandmother.

Reverend Gailbraith is a local religious figure who holds the Culduie crofters in low esteem. He actively dislikes the Macrae family, viewing their religious understanding as inadequate and Roderick's character as fundamentally flawed from an early age.

William Gillies is the local schoolteacher who educated Roderick. Unlike the rest of the community, he recognizes Roderick's exceptional intelligence and actively encourages John Macrae to allow the boy to continue his education beyond the basic requirements.

Carmina is a resident of Culduie who lives near the Macrae and Mackenzie families. She closely observes the goings-on in the village and catches Roderick engaging in inappropriate behavior outside her home.

Kenneth is a crofter in Culduie who takes charge of the immediate aftermath of the violence at the Mackenzie home. He locks Roderick in an outbuilding to prevent further chaos while awaiting the authorities.

Una is John's late wife and the mother of Roderick and Jetta. Her death in childbirth serves as a devastating catalyst for her family's decline, plunging them into a grief from which they never recover.

Dr. Munro is a medical examiner who questions Roderick to determine his state of mind. He relies on traditional medical knowledge and rejects the newer, experimental theories of criminal psychology proposed by the defense.

Peter is Lachlan's cousin and a vocal supporter of his kinsman. He views Roderick as exceedingly evil and dismisses John Macrae as a religious idiot, highlighting the deep community divides.

Aeneas is Lachlan's brother. He acts as an enforcer for his sibling during village disputes and openly testifies that Roderick is entirely mad.

Burnet serves as the fictionalized compiler of the novel's documents. He claims to have discovered the trial records while researching his own family history, presenting them to the reader with minimal editorial interference.

Ishbel is a young woman who spends time with Flora Mackenzie. She witnesses the awkward interactions between Flora and Roderick, later providing insight into Flora's true feelings about the boy.

Calum is the parish constable before Lachlan Mackenzie takes the post. He attempts to mediate disputes fairly but struggles under the pressure of the Factor's financial demands.

Lord Middleton is the wealthy nobleman who owns the land Culduie sits upon. He represents the peak of the local social hierarchy, viewing the land and its wildlife merely as resources for his personal sport.

Duncan is a Culduie crofter who benefits when Lachlan Mackenzie reduces the Macrae family's land allotment. Despite the contentious circumstances, he attempts to share his profits with John Macrae to maintain peace.