Warning: SPOILERS ahead for Dexter: New Blood episode 1.
Dexter has taken a long gap in murdering at the start of New Blood, with Deb’s death having largely prevented his serial killing after season 8. Dexter: New Blood episode 1 depicts the titular main character's routine in Iron Lake, New York following the original show's divisive finale, revealing that he hasn’t killed anyone in almost 10 years, since mercifully pulling the plug on Deb’s life support. After faking his death and living under a new alias, Dexter has learned to suppress the urges of his Dark Passenger. The voice in his head, which was originally portrayed by his adopted father Harry Morgan (James Remar), now takes the form of his sister Deb (Jennifer Carpenter) in New Blood.
In a shocking resolution for one of Dexter’s most beloved characters, the season 8 finale ended with the death of Debra Morgan, Dexter’s adoptive sister and closest human connection. After he had seemingly restrained Oliver Saxon enough for him to leave for Argentina and let Deb handle his capture, Oliver got hold of a gun and shot her. Following time in the hospital during which she seemed to be on the mend, Deb suffered a major blood clot that left her permanently brain dead. In the final moments of Dexter's generally hated series finale, Dexter unplugs her life support, carries her to his boat, and dumps her into the ocean covered in a white sheet. The series ends with the lead driving straight into the eye of the hurricane. A lot of characters died throughout Dexter, but no death hit him like Deb's.
There’s a moment in New Blood episode 1 when Dexter is eating breakfast alone at his table, with his subconscious manifestation of Deb laying bored across the table. It’s a heartbreaking scene because it’s a reminder that Deb should just be casually lounging around the cabin, and truly reveals how comforting her presence is to Dexter’s sanity. Deb explains that she’s glad she went first because she never could have survived without him—though this is Dexter's psychological manifestation—when it’s clear that he truly can’t survive without her either. "Deb" likely replaced Harry shortly after her character's death. Reminding Dexter of this tragedy at every waking moment has prevented him from killing and dooming another person he loves to the same fate.
Deb was one of the only people that he knew genuinely loved him, and was the one person throughout his life that he always had at his side. Even in the original series premiere episode, Dexter says if he could feel genuine human emotion and connection, he would want to feel that for her. Deb's death in Dexter season 8, which was partly due to his own mistake, profoundly changed his outlook on his life and actions. Deb was the closest person to him, sacrificing her own morality while putting her career at risk to keep Dexter safe. If he let her die, how could he be sure it wouldn't happen to his son Harrison? Deb's death was the straw that broke the camel's back, and signaled to Dexter that his Dark Passenger ultimately did more harm to his life than good.
This is also where the manifestations of Deb and Harry differ in their duties as the Dark Passenger. Harry was a reminder of what Dexter could be if he followed "The Code", while Deb is a reminder of what he can't be and all the worst things that he's ever done. All Deb has to do to stop Dexter’s serial killer urges is name their loved ones who died, and pull out the bullet that took her life. With Harrison back in Dexter’s life, Deb’s influence is waning—especially since he finally killed again for the first time in New Blood’s premiere episode. As Dexter grapples with returning to his kill room and needing to protect Harrison at all costs, Deb may actually be more helpful to him in understanding why he may need to kill in order to keep Harrison alive.