This episode is the first to focus on the new character Ezri Dax (played by Nicole de Boer), who was introduced in the first episode of the seventh season. A member of the Trill species, Ezri is the new host for the symbiotic lifeform previously carried by main character Jadzia Dax; Jadzia was killed at the end of the sixth season to make way for actress Terry Farrell to depart the show, and the Ezri character was written to replace her.
This episode marks the debut of Ezri Dax (played by Nicole de Boer), a new Trill character who takes over from Jadzia Dax, played by Terry Farrell, in the seventh season.  Jadzia died at the end of the sixth season to make room for Farrell's departure, and Ezri was created to fill her role.