When Dexter Morgan was three-years-old, he and his brother Brian, who would later become a serial killer himself, witnessed their mother being murdered by a drug dealer with a chainsaw. At an early age, Dexter started to kill and dismember animals. His adoptive father Harry, a homicide detective, realized that Dexter was a potential serial killer. After attempts to curb Dexter's violent behavior failed, Harry concluded that Dexter's need to kill could not be suppressed--only focused. Making the best out of a bad situation, Harry decided to train Dexter as a vigilante, to solely hunt and kill other murderers (arguably making society safer). To avoid arrest, Dexter follows a set of guidelines, the Code of Harry. The irony of Dexter's life is that he worked closely and affably with homicide detectives and a forensic specialist. Dexter, himself a blood spatter analyst, collected, processed, and interpreted evidence for the Miami Metro Police Department. He often used his ability to access police databases or tamper with evidence, for his personal advantage. Dexter generally disposes of his victims by dismembering them, putting the parts into black trash bags, and dropping the bags off his fishing boat—The Slice of Life—into the ocean. He has a younger adoptive sister Debra Morgan.

James Doakes is a strong-willed sergeant within Miami Metro Homicide. He acts as both a detective and a supervisor at any given crime scene. He often partners with Detective Angel Batista and Officer Debra Morgan, while dealing with forensics on scene, namely Dexter Morgan and Vince Masuka. His direct supervisor (and close friend) is Lieutenant Maria LaGuerta, with whom he had previously been partners for over two years. He has always had a bad feeling about Dexter, but can never quite place it.

Vince Masuka can be seen as the comic relief of the show. His role in the overall plot is to typically add perverse jokes and to lighten serious situations. He usually makes some sort of humorous comment regarding the way victims are found. He is the lead forensics specialist for the Miami Metro Police Department, working alongside Dexter Morgan as his partner. He offers the homicide team constructive insight with his own perverse style and assists the department with crucial information in cases.

Debra "Deb" Morgan is not shy and can be headstrong at times. Despite the tough persona she presents, she's sensitive and caring inside although she doesn't readily express it. Debra's ability to be "one of the guys" allows her to be effective at her job. Suspects can anger her, especially when something violent happened to someone close to her. She's a lot of things that Dexter is not: crass, impulsive, emotional, trusting, immature, and thus serves as a perfect foil for Dexter. She highlights his shortcomings in his attempts to be "human." Although she's often frustrated with Dexter's inability to open up to her, she defends him on more than one occasion.

Maria LaGuerta leads almost every investigation with Sergeant James Doakes and Angel Batista supervising the crime scenes under her. She handles the political side while her detectives do the dirty work. She is very determined to maintain her position no matter the cost, such as blackmailing Captain Matthews into promoting her. She is also very manipulative, telling Matthews that she would help make the call girls case go away, but instead used Debra by having her dig information on the case that would connect back to Tom and sold him out to the board and had him forcibly put into retirement to gain his position.

Angelo "Angel" Batista is basically good-natured, with a dry sense of humor. He is also totally honest, at the request of his beloved father on his deathbed. This later lands him in trouble, as it drives him to expose Doakes' killer instincts to internal affairs and, after having an affair with another woman, confesses his actions to his wife who ends their relationship, After three months of being on his own, Batista turns to spiritual enlightenment to alleviate his pain.

Matthews frequently clashed with Lieutenant Maria LaGuerta. At one point, he temporarily installed another Lieutenant in her place when she made it public about the mistakes he made during the Ice Truck Killer Case publicly embarrassing him. Despite his flaws and his own political ambitions, Matthews was a good man at heart, truly caring about the officers under his command and their performance in the field.