5. Isaak Sirko - Season 7
Isaak Sirko is the head of the Koshka Brotherhood, who comes to Miami to avenge the death of his lover, Victor, who was killed by Dexter. Sirko is a cold, brutal, killing machine, leaving a trail of bodies in his wake, which Dexter quickly notices. Dexter tries to have Isaak killed by a rival Columbian gang, but underestimates Isaak's skills. Isaak kills the men with ease but is arrested, nonetheless. The two form an interesting relationship, after Isaak is let out of prison and attempts to kill Dexter, and the two men develop a mutual respect for one another. Isaak tells Dexter that, "under different circumstances, we could have been good friends." Isaak is eventually killed by assassins, and Dexter dumps his body at sea so that he may rest with Victor. Though he may not have left the show as a villain, he remains one of the most intriguing antagonists in the entire series. He shows no mercy when hunting Dexter, yet is able to overcome his need for revenge in order to conquer a common enemy. Aspects of his character are revealed slowly and interestingly throughout the season, and his final scene on the boat with Dexter remains one of the most memorable of season 7.
Memorable Moment: Forcing a man to write a fake suicide note, confessing to a crime he didn't commit, to save one of Isaak's men.