Debra Morgan is Dexter's step-sister.
They became related when Dexter was adopted by Deb's father, Harry, who instantly developed a bond with his new son. Deb grew of very jealous of it.
Debra is a detective in the Miami Police Department. At one point, she got engaged to a man that ended up being the notorious murderer known as The Ice Truck Killer. That's never fun.
This detective as a seriously dirty mouth and a very aggressive, up-front attitude. She's far from shy.
Deb: (excitedly) A baby? a motherfucking rolly-poly, chubby cheeked shit machine? Are you kidding me?
Dexter: I've never heard it described in quite those words before, but yeah.
Debra Morgan: Why is it we never talk brother-sister stuff?
Dexter Morgan: Our dad was a cop, you're a cop, I work for the cops... for us, this is brother-sister stuff.