By day, Dexter is a forensic blood spatter analyst for the Miami-Metro Police Department. In his own time, Dexter is a serial killer. He was taught by his adopted father, Harry, to only kill other killers who have escaped the traditional legal system, or were never suspected in the first place.Dexter Morgan is driven to kill to satisfy an inner voice he calls "the Dark Passenger." When that voice can no longer be ignored, he "lets the Dark Passenger do the driving." When talking about his 'work' in the TV series he explains the code as, "My intention was never to save lives, but save lives I did."
In Dexter in the Dark, the third novel of the Dexter series, it is implied by a murderous cultist that the Dark Passenger is an independent agent inhabiting Dexter rather than a deviant psychological construction. The Dark Passenger is supposedly the offspring of Moloch, a god worshiped in Biblical times. Dexter is momentarily frightened into researching supernatural explanations for his homicidal urges.
Dexter considers himself emotionally divorced from the rest of humanity; in his narration, he often refers to "humans" as if he is not one of them. Dexter makes frequent references to an internal feeling of emptiness, and says he kills to feel alive. Dexter claims to have no feelings or conscience and that all of his emotional responses are part of a well-rehearsed act to conceal his true nature. He has no interest in romance or sex; he considers his relationship with Rita to be part of his "disguise". In the show he shows considerably more attachment and fondness for Rita than in the books. For example, in the first season he is afraid to have sex with her because he might lose her as a result, and because Rita's ex-husband was abusive to her.
There are holes in Dexter's emotional armor, however. He acknowledges loyalty to family, particularly his late adopted father: "If I were capable of love, how I would have loved Harry." Since Harry's death, Dexter's only family is his sister, Deborah, Harry's biological daughter. At the end of the first novel, Dexter admits that he cannot hurt Deborah or allow Brian to harm her because he is "fond of her".
Dexter likes children, finding them to be much more interesting than their parents. The flip side of this affection is that Dexter is particularly wrathful when his victims prey on children. In Dearly Devoted Dexter, Dexter realizes that Rita's son Cody is showing the same signs of sociopathy as Dexter himself did at that age, and looks forward to providing him with "guidance" similar to that which Harry provided him; in his way, he sees Cody as his own son. This also gives him a reason to continue his relationship with Rita; as of Dearly Devoted Dexter, he is engaged to her because of a misunderstanding (Rita finds a ring that came off a severed finger in Dexter's pocket). In the beginning of the third book it has been shown that Cody is not the only one with violent impulses, as both children pressure Dexter to "teach" them. Dexter has come to accept his role as stepfather to both children very seriously in Dexter in the Dark, albeit in his typical fashion. For example, while on a stakeout, he begins to wonder if Cody had brushed his teeth before bed and if Astor had set out her Easter dress for photo-day at her school. These thoughts distract him while he is waiting for an intended victim, which thoroughly annoys him.
Animals don't like Dexter, which can cause noise problems when Dexter stalks a victim who has pets. He is quoted as once having a dog who barks and growls at Dexter until he is forced to get rid of it, and a turtle, which hides in its shell until it dies of starvation rather than have to deal with Dexter.
