Ezri Tigan, was born on New Sydney in the Sappora system. Her mother, Yanas Tigan, was a successful mining magnate on Sappora VII, and she and Ezri disagreed on many occasions. Her father was often. Ezri's closest family member was her younger brother, Norvo (who gave her the nickname Zee), while her older brother, Janel, was someone whom she looked up to. Ezri dreamed constantly of ways to escape her family, and eventually managed to do so by joining Starfleet.

In 2375, Ezri was an ensign serving as an assistant ship's counselor aboard the USS Destiny when it was dispatched to Deep Space 9 on a medical emergency, to carry the Dax symbiont back to Trill following the death of its previous host, Jadzia Dax. En route, the symbiont took a turn for the worse, and, as the only Trill aboard, though one who had decided against joining, Ezri had little choice but to undergo the joining procedure, with only a fifteen-minute lecture from the ship's non-Trill surgeon to prepare her.

Following the joining, the Destiny continued onward to Trill, only now to deliver Ezri so she could meet 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.

Seeking some sort of link to the person she was now, Ezri sought out Benjamin Sisko, a friend of Dax's previous two hosts, to see if he could help her through the difficult process of integrating eight lifetimes of experience. Sisko asked her to join his crew as Deep Space 9’s counsellor and she was promoted Lieutenant Junior Grade. Dax stayed on DS9 till the end of the Dominion war.