My notes say INFJ. How anyone could see him as ST is beyond me.
I'm absolutely certain he's an N. He's always (darkly) daydreaming, caught up in his own little web of thoughts. Most of the cops are bound to Sensors, so see how they notice completely different things about cases than Dexter does. Sensoring - being present and grounded is what 'drains' his battery. Anytime he hasn't gotten time to himself and his projects, he loses his attention to details and slips up. He quite literraly becomes clumsier, which also points toward N.
Introvert - he prefers to work alone, recharges his batteries with solo projects. Drains his batteries when putting on his "mask" and attempting to connect with people.
Judger - a neat freak, very punctual, very precise, believes in control.
Feeler - were Dexter a Thinker, he wouldn't even consider himself a monster. All he talks about is how he has no feelings and how he isn't human, and it's getting ever clearer that he's just telling himself these things, and he's in denial. He would like to be a Thinker, because then he might not have feelings distract him. The truth is, he is a romantic. If that daydream about him being some rogueish, cloaked vigilante didn't just scream Idealist, I don't know what does. He wants to be an NT, because being an NF seems to be his achilles' heel. It could also be his greatest strength (depending on how well the rest of the show is written).
For the sake of contrast his brother Brian is the clear Thinker, and he doesn't have -any- of Dexter's emotional complications.
So I'm convinced Dexter is INFJ aka "the protector".