Orphaned at the age of three, Dexter Morgan was adopted by a Miami police officer named Harry Morgan. Harry recognized Dexter's sociopathic tendencies and taught him to channel his gruesome passion for thrill killing towards people who deserve it. Dexter's victims must be killers themselves; people who have killed the undeserving (usually multiple times) with no remorse. He must also have proof that the criminals are guilty before he takes action.

Flashbacks throughout the series show how Harry, who died several years before, instructed Dexter in the art of appearing normal and covering his tracks. He follows Harry's instructions closely, calling them "The Code". Like many serial killers, Dexter keeps trophies from his kills; before dispatching a wrongdoer Dexter makes a small incision on their right cheek with a scalpel and collects a blood sample which he preserves between two glass slides. He stores his collection of blood slides in a box concealed inside his air conditioner.

Although his drive to kill is unflinching, Dexter is, through extensive instruction from Harry, able to fake normal emotions and keep up his appearance as a socially responsible human being. To hide in plain sight and to erase his own crimes, Dexter works as a blood spatter analyst for the Miami Metro Police Department. Dexter also uses his calculated charm to become well liked by all of his colleagues, except for a few who think something is odd about him.

Despite not having any "real" feelings, he feels affection towards Harry’s biological daughter and his adoptive sister, Debra. Debra makes her jealousy known of the perceived preferential treatment Harry gave Dexter, but is unaware of Harry's training or Dexter's tendencies. He also dates a woman named Rita in order to appear normal. Rita is psychologically damaged from past rape and abuse by her ex-husband, Paul, and is too distraught to want to be in an intimate relationship. This suits Dexter, who believes he is unable to fake intimate emotions. He also is fond of Rita’s two children, Astor and Cody.