14. Louis Greene - Seasons 6 & 7
Louis might very well be the show's biggest missed opportunity. We first see him in season 6, as a lab intern under Vince Masuka. He had an intriguing creepiness of his own and appeared to have a special interest in Dexter that bordered on obsession. He and Dex played an interesting game of cat and mouse as audiences tried to decipher what his true motivations were, and what secrets of Dexter's he may have known. At the end of season 6 we're led to believe that he may be the "Big Bad" of season 7, and early fan speculation suggested that perhaps he was Dexter's nephew, the son of Brian Moser, the Ice Truck Killer. Unfortunately Louis ended up being nothing more than a scorned video game developer, mad at Dexter for mocking him. I can't help but feel that the writers had big plans for him that fell short once they started developing season 7, and decided to ultimately kill him off rather early (and in a pretty unspectacular fashion) to focus on other characters.
Memorable Moment: Showing up in Dexter's apartment after Dex threatened him to stay away. Ultimately not very smart, but a ballsy move nonetheless.