Ezri Dax, formerly Ezri Tigan, was a joined Trill, the ninth host of the Dax symbiont and served as counselor aboard Deep Space 9, beginning in 2375. After the previous host of the Dax symbiont, Jadzia Dax, had died, the Dax symbiont was rushed to Trill on the starship USS Destiny. On the Destiny the symbiont's status deteriorated and Ezri, as the only unjoined Trill on the ship, had to undergo the procedure of joining.
Following the joining, Ezri returned to Trill with the Destiny, and met with members of the Symbiosis Commission's Evaluation Board, who gave her counseling on how to deal with her new memories, but they were unable to help her further. With her friends and family having difficulty adjusting to her changed personality, and her own confusions about who she really was, Ezri took a leave of absence. She traveled to Earth and met a friend of the two previous hosts, Benjamin Sisko. Together with him, she uncovered the Orb of the Emissary, and restored the Prophets to the Celestial Temple.
She then took a commission on Deep Space 9 as counselor and remained there for the rest of the Dominion War.