As “Dexter” fans no doubt recall, Deb died at the end of “Dexter” Season 8. Because Carpenter is returning, that leaves two realistic possibilities for how she might reprise the character. It could be via flashbacks, but on the rare occasions when “Dexter” does use them, the flashbacks usually show Dexter and Debra in their formative years, not as adults. As many Redditors have pointed out, it’s much more likely that Debra will serve a similar role as her and Dexter’s late father, Harry Morgan (James Remar).
Harry Morgan was the first person to discover Dexter’s true nature, and he helped his adopted son devise a way to ethically live with his “Dark Passenger.” Harry also died before the series took place. On the show, he appeared in almost every episode as a figment of Dexter’s imagination, to guide him.
However, Harry’s role changed over the course of the series. Harry died before Dexter really started his serial killing career, so Harry never saw the worst things Dexter did — like murdering fellow officer James Doakes to keep his secret hidden, allowing Deb to murder LaGuerta for the same reason, or allowing his serial killer lifestyle to destroy Deb. As the show went on, Harry’s advice became increasingly obsolete, and Dexter lost his way. Heading into the revival, Dexter needs a new mentor. That needs to be someone who knows what he is, and after Harry, nobody knew him better than Deb.
So, Deb replacing Harry as Dexter’s mentor makes the most sense, but until the revival airs, we’ll have to wait and see what happens.