Ezri Dax is a member of the Trill joined species, consisting of the humanoid host Ezri, and the symbiont Dax. Ezri has a mother, Yanas Tigan, and two brothers - Norvo Tigan and Janel Tigan - who live in the Sappora System. The Tigans own and operate a family mining business, the sixth largest in the sector.

Ezri Tigan was joined with the Dax symbiont after the death of Jadzia in 2374. While en route to Trill aboard the U.S.S. Destiny, the Dax symbiont took a turn for the worse and needed to be joined immediately. Ezri Tigan was the only available Trill host on board and had to be implanted with the Dax symbiont. Ezri did not go through the Trill Symbiosis Institute initiate training, but received an emergency 15-minute lecture from the surgeon who performed the implantation. As a result, she has difficulties coping with the symbiont's multiple personalities. As with Trill tradition, the symbiont name replaces the new host's family name and she became Ezri Dax.

In 2375, Starfleet Medical waived Ezri's remaining training period and promoted her to full counselor. She was promoted to the rank of Lieutenant junior grade.

The Dax symbiont has had nine hosts: the ninth is Ezri, the eighth host was Jadzia, the seventh was Curzon, the sixth Joran, the fifth Torias, the fourth Audrid, the third Emony, second Tobin, first Lela. The Dax symbiont has been married five times: three as a bride, two as a groom, and has had nine children: five as a mother, and four as father. Tobin was an engineer. Emony was an olympic gymnast. Torias was killed in a shuttle accident. Curzon was a Federation-Klingon ambassador. Jadzia was a Starfleet science officer. Ezri is a Starfleet counselor.

Starfleet History
Attended Starfleet Academy
2374 Posted to U.S.S. Destiny, Assistant Ship's Counselor.
2375 Joined with Dax symbiont.
2375 Assigned to station Deep Space Nine, Station's Counselor, Promoted to Lieutenant, junior grade.

Ezri Dax was first seen in "Image in the Sand" (DS9).