Lt. Maria Laguerta
Lt. Maria Laguerta is in charge of the Miami Police Department. She's the boss of Dexter, Debra and the entire crew.
Laguerta was initially relieved of her post in the first season, but plotted to take it back from the new boss by sleeping with her fiance. Indeed, Maria isn't afraid to get dirty in this business.
She had a romantic history with Sergeant Doakes, which was revealed in season two; and with attorney Miguel Prado, which is revealed in season three. Laguerta seems like a solid Lieutenant, treating her charges fairly.
She actually got married to Angel, after briefly dating, on season four.