Warning: Wikipedia contains spoilers.
A Trill Starfleet officer, Ensign Ezri Tigan was assigned to the U.S.S. Destiny as assistant ship's counselor upon leaving Starfleet Academy. She had no interest in becoming joined with a Trill symbiont.
After Lt. Cdr. Jadzia Dax was killed in action aboard Deep Space Nine, the Dax symbiont was saved by Dr. Julian Bashir and sent to Trill aboard the Destiny for re-joining. However, the symbiont became gravely ill during the flight, and as the only Trill on board, Ensign Tigan was asked to join with Dax in order to save its life. Ezri Dax is Dax's tenth host.
Upon joining, Ezri was assigned to Deep Space Nine and granted a field promotion to Lieutenant (j.g.). She served as counselor, but also distinguished herself as a science officer (in Jadzia's footsteps) and in combat in the Dominion War.
Althogh Jadzia had been married to Cdr. Worf, Ezri chose not to pursue that relationship; instead, she began a liaison with Dr. Bashir.