Who discovers Dexter's secret in season 7?

LaGuerta discovers Dexter’s secret in season 7 by concluding that he is the Bay Harbor Butcher. Maria LaGuerta, played by Lauren Vélez, becomes suspicious after the Bay Harbor Butcher case is reopened and new evidence points toward Dexter.

LaGuerta finds evidence connected to Dexter’s blood-slide collection and notices similarities between his methods and the murders attributed to the Bay Harbor Butcher. Her investigation places her directly against Dexter, although the truth is difficult to prove and is complicated by Dexter’s efforts to conceal his identity.

The wording can cause confusion because other characters learn parts of Dexter’s secret at different times. Debra Morgan discovers that Dexter is a serial killer near the end of season 6, while James Doakes uncovers that Dexter is the Bay Harbor Butcher in season 2. In season 7, however, LaGuerta is the police official who independently identifies Dexter as the killer.

LaGuerta’s discovery drives the season’s final conflict. She attempts to expose Dexter, while Debra becomes caught between protecting her brother and stopping him from killing again.