Warning: Wikipedia contains spoilers.
Jadzia Dax is a joined Trill. After completing her training at Starfleet Academy, Jadzia returned to Trill where she applied to the Trill Symbiosis Commission for joining. She was the only Trill in history to successfully complete the program after having failed once; her re-application was sponsored by Curzon Dax, and when she completed the program, she was given the Dax symbiont after his death.
She served as science officer aboard the space station Deep Space Nine under the command of Benjamin Sisko, who had been a friend of Curzon. Initially very reserved, she soon acquired a reputation as a bonne vivante with an especial interest in Klingon culture. On one occasion she disobeyed Sisko's orders to fulfil a blood oath to Kor, a Klingon captain whom Dax had known in a previous host, and help to kill an enemy of his.
On one occasion, the Dax symbiont was removed from Jadzia and kidnapped by an unjoined Trill, Verad, who believed he had been cheated of being joined. He was briefly Verad Dax before he was apprehended; Dax was rejoined with Jadzia.
Jadzia was threatened with the removal of Dax on another occasion, when it was discovered that a previous host, Joran Belar, had been psychotic.
At one point Jadzia faced ostracism from Trill culture because she began a relationship with a Trill Scientist, Lenara Kahn. The previous hosts of both symbionts, Torias Dax and Nilani Kahn, had been married, so Jadzia and Lenara were violating a Trill taboo against reassociation. Jadzia and Lenara broke off the relationship rather than face the taboo.
Although she was romanced both by Dr. Julian Bashir and Quark, she fell in love with and married Lieutenant-Commander Worf after he transferred to DS9. Jadzia was carrying Worf's child when she was killed during the Dominion War by Gul Dukat, who was possessed by a pagh wraith and was attempting to destroy one of the Tears of the Prophets.
While being returned to Trill, Dax nearly died too; the emergency forced its implantation into Ezri Tigan, who was not prepared for joining. She would later transfer to DS9.