Dexter is a blood spatter analyst for the Miami Metro Police Department who lives a double life as a vigilante serial killer. Guided by an inner voice he calls the "Dark Passenger," he adheres to a strict moral and behavioral code taught to him by his adoptive father. He fakes normal human emotions and social interactions to blend in with his colleagues and peers, masking his fundamental lack of empathy.

Deborah is a vice cop with the Miami Metro Police Department who desperately wants a transfer to homicide. She is brash, loud, and unpolished, which often hinders her career advancement despite her strong investigative instincts. She relies heavily on her adoptive brother for advice and insight into her cases, trusting his seemingly preternatural understanding of criminal behavior.

Harry is a former Miami police officer who adopted Dexter after responding to a traumatic crime scene. Disillusioned by a justice system that often lets predators go free, he recognized Dexter's violent tendencies early on and taught him a strict code. This code directs Dexter to only target other killers, ensuring his urges serve a form of vigilante justice while keeping him hidden from the law.

Lieutenant LaGuerta heads the Miami Metro Police Department and prioritizes departmental politics over rigorous investigative work. She is highly focused on media optics and her own career advancement, often rushing to make arrests that look good to her superiors and the public. She creates a difficult work environment, frequently bullying subordinates she dislikes while flirting with those she wants to manipulate.

Brian is a man profoundly scarred by witnessing a brutal cartel murder during his early childhood. Left without a supportive family structure or adult guidance after the trauma, he grew up entirely devoid of empathy and consumed by violent impulses. He fully embraces the darkness within him, representing the devastating cycle of unchecked childhood trauma.

Rita is a survivor of severe domestic abuse who is focused on rebuilding a stable life for herself and her children. Her traumatic past with an incarcerated ex-husband has left her wary of physical intimacy, which makes her an ideal partner for Dexter. She is hardworking, resilient, and deeply committed to providing a safe home for her family.

Vince is a crime lab colleague of Dexter's at the Miami Metro Police Department. He dresses casually in colorful bowling shirts and is known for a slightly off-putting smile and boundary-pushing jokes. Despite his sometimes offensive humor, he shares an easy camaraderie with both Dexter and Deborah at grim crime scenes.

Angel Batista is a Cuban American police officer working with the Miami Metro Police Department. He represents the multiethnic makeup of the city's law enforcement and frequently interacts with Dexter and Deborah as they investigate local homicides.

Sergeant Doakes is a cold-eyed, intense officer at the Miami Metro Police Department who harbors a deep, instinctual mistrust of Dexter. Unlike most colleagues who find Dexter merely quirky, Doakes seems to recognize a hidden darkness in the blood spatter analyst and makes his suspicion openly known.

Captain Matthews is a high-ranking official in the Miami Metro Police Department who oversees Lieutenant LaGuerta. He is stern, attentive to protocol, and quick to threaten disciplinary action when his subordinates prioritize media spectacle over proper police work.

Daryll Earl McHale is a petty thief and violent domestic abuser with a long criminal record. Lieutenant LaGuerta eagerly arrests him for a series of high-profile murders, though Dexter and Deborah immediately recognize that he lacks the intelligence and precision necessary to execute the methodical crimes.

Astor is Rita's young daughter, who witnessed significant domestic violence perpetrated by her father. The trauma has left her quiet and withdrawn, prompting Dexter to act as a supportive presence in hopes of preventing her from continuing a cycle of violence.

Cody is Rita's young son, who also suffers from the traumatic effects of his father's violent behavior. Like his sister, he is quiet, but he slowly benefits from the consistent, supportive attention Dexter provides during family outings.