Dax was a Trill symbiont first introduced in the Star Trek: Deep Space Nine pilot episode Emissary.
Born in the Earth year 2018 on Trill, Dax spent the first century of its life growing to maturity in the Caves of Mak'ala on Trill. In the early 22nd century Dax was joined to a woman named Lela Yurani, becoming Lela Dax in the process.
During this first joining Dax would have a political opponent in the Odan symbiont's host, but later Odan would defend her against treason charges when Dax tried to contact an alien race without authorization.
Afterwards the successive hosts Dax would go on to play a major role in both Trill society, and the United Federation of Planets as well. Over the next two centuries in its various hosts Dax would work in a variety of roles. As Curzon Dax he became a mentor to Starfleet officer Benjamin Sisko, who would become a major figure in fighting the war against the Dominion. The next two 24th century hosts Jadzia and Ezri would serve as Starfleet officers, helping to defend the Federation against the Dominion and the Borg.
Over its lifetime as a joined Trill, Dax came to know a large number of prominent people, including Jonathan Archer, Malcolm Reed, James T. Kirk, Skon and his son Sarek, Hikaru Sulu, Nyota Uhura, Christopher Pike, and Doctor Leonard McCoy, among many others.