Imagine
Four times Deb sees his mask almost slip, and the one time it actually does. And how Dexter plays his sister.
The body is cut up into small pieces, and they suspect it was done while the vic was still alive. Dex's face is in a slight, childish, 'eww, gross' expression. They have seen worse; at least the guy was allowed to bleed out. She can hardly believe the things her species does, sometimes. The deaths usually seem so pointless to her. Why do they have to die? She is taking a statement from the person who found the vic--his girlfriend. She glances up from her pad, intending to look at the girlfriend, but something draws her eyes over the other woman's shoulder.
Her brother in everything but blood is alone near the body. There are people all around, but he still looks so separate--removed, as if this doesn't affect him at all. The look on his face is caught, reminding her of something saved from falling to the ground. Just inches above disaster. He is struggling to regain his detached interest--detached because they have to be, to not go insane.
Behind that she see a black abyss.
She's not the quoting type, but 'When you look into the abyss, the abyss also looks into you.' sounds pretty damn accurate right now. Dex looks back at her and he smiles, and Deb convinces herself that she imagined the whole thing, again.
She's sure imagined it. When Dex found out the body was Doke's an expression of elation and worry and arrogant 'I know something you don't know' and 'I've won again, and you don't even realize it' most certainly did not cross his face.
That's not the Dex she knows, and how can she have grown up with someone and not know them?
She wakes up restrained by plastic wrap, bound to a cold metal table and gagged in the same way. Above her, Dex is holding the wrist of the man who she now knows is the Ice Truck Killer. They hold a knife, barely and inch above her heart.
She will never admit that her first thought is that they are killing her together.
Her second is correct, that Dex has saved her. The Ice Truck Killer flees. She is sure, sure, sure, that she imagines the look of regret on Dex's face when he turns to free her.
The first murder, they don't know it's a serial killing. So she doesn't know why Dex looks so interested; it's just another case. They'll solve it and move on. Except that he is excited, and she's never seen that before. There is an ominous feeling in her stomach, and she thinks her death is standing behind her.
In the back of her mind, she knows it's actually right in front of her, and wearing her brother's face like a Halloween mask.
Deb remembers one thing, and one thing only from her childhood. It is three days after Dad brought Dex home, and she doesn't know what to make of his quiet nature and blank eyes.
Then she finds him holding down the neighbor's cat, squeezing it's throat, blood rushing down his hands in little rivulets, and a look of utter, exhilarating bliss on his face.
She hardly looks at him for months, until her young minds files it away under traumatic experiences and leaves it to rot. It is called up again the night after Dad dies, and she refuses to believe it is anything other than a stress-created nightmare.
This is the only time Deb ever sees the thing that poses as Dexter. She has managed to convince herself that this is a bad dream, and that makes it easier to dismiss all the other signs she collects over the years.
Dexter knows his mask is a little too perfect. It occasionally tries to emulate the things his dark passenger would be feeling, if it were human. He catches it, always, but maybe one day he won't. He's tempted to do it, just to see the reactions. The act is tiring, but he has an impossible mission, one he will not give up on: To clean the city of criminals. This keeps him going.
He knows Deb has convinced herself that he is normal, and that makes it so much more rewarding to let her glimpse his true self. He gets as close to amusement as he can when he watches her trying desperately to convince herself that her own brother isn't exactly the same as the Ice Truck Killer.