Dax is a xenomorphic character in the fictional Star Trek universe. The Dax life form is a symbiont—one that lives inside & bonds to Trill humanoid hosts. The first appearance of a Trill was in the episode "The Host" from the fourth season of Star Trek: The Next Generation; in that early version, the personality of the being was entirely that of Odan, the symbiont within the host, whereas the personalities of the Trills in Star Trek: Deep Space Nine are a blending of the symbiont Dax and its sequential Trill hosts. Two of Dax's hosts, Jadzia Dax and Ezri Dax, appear as major characters in Star Trek: Deep Space Nine. Others are only seen in flashbacks and when taking over others' bodies in the third season episode "Facets". Dax's hosts also appear in spin-offs such as Star Trek: The Human Frontier and The Lives of Dax.
Throughout the franchise's timeline, Dax has been joined with four men and five women, living for more than three hundred years in total. Dax has been present for many important events in Star Trek history, such as the negotiation and signing of the Khitomer Accords (while joined with Curzon), the discovery of the Bajoran wormhole (while joined with Jadzia) and the end of the Dominion War (while joined with Ezri).
Official hosts
Lela Dax
Lela Dax is the first host of the Dax symbiont. Lela appears in the "Facets" Deep Space Nine episode where her consciousness takes over the body of Kira Nerys (Nana Visitor) at Jadzia's zhian'tara ceremony. She is the first Dax incarnation to which Jadzia speaks. Lela is shown as very sociable and easy-going. She comforts Jadzia and dispels her worries as Jadzia was anxious of what she might experience at the ceremony. When talking about herself, Lela says she was one of the first women to be elected to the Trill legislature. She says she took the habit of clasping her hands behind her back to curb excessive hand gesturing while speaking publicly. Jadzia finds she has inherited this behavior.
Tobin Dax
Tobin Dax is an engineer and mathematician in the Trill history and is the second host of the Dax symbiont. In the "Facets" episode, Tobin possesses Miles O'Brien (Colm Meaney). He is shown as extremely timid and shy in social situations. He is very nervous, biting O'Brien's fingernails and continuously making apologies for no real reason. Jadzia tells him she was able to recall his own proof of Fermat's Last Theorem and was fascinated by its original approach.
In the same episode, Curzon Dax tells Jadzia that at his own zhian'tara when his friends and family temporarily embodied past Dax hosts, he gleefully got Tobin's host roaring drunk.
In "The Siege", Jadzia uses Tobin's knowledge of sub-impulse thrusters to repair one on an old Bajoran fighter. She describes Tobin as having "barely a sex life and no imagination, but he knew phase coil inverters like no one else".
In the non-canon short story "Dead Man's Hand" (written by Jeffrey Lang and collected into The Lives of Dax), Tobin Dax is involved in the creation of an early transporter and encountered the Vulcan scientist Skon (Spock's grandfather) during an altercation with enemy Romulans.
He is also featured in the novel Star Trek Enterprise : Rise of the Federation - A Choice of Futures as the leader of a team from the Cochrane Institute working on integrating multi-species technology in an emerging Federation.
Emony Dax
Emony Dax is the third host of the Dax symbiont. She is an Olympic gymnast. In "Facets", she inhabits the body of Leeta (Chase Masterson). Emony thanks Jadzia for finding such a splendid host for her as Leeta's body is well-trained and flexible, able to perform acrobatic tricks. She tells Jadzia that she was afraid her concentration and physical coordination would be impaired due to the join, but it turned out just the opposite.
In the "Trials and Tribble-ations" DS9, Jadzia recalls that Emony traveled to Earth once to judge a gymnastics competition at the University of Mississippi where she met a young Leonard McCoy. She says Emony had a feeling that Leonard would become a doctor: "He had the hands of a surgeon". The story of their meeting appears in the non-canon short story "Old Souls" (written by Michael Jan Friedman and collected into The Lives of Dax).
Audrid Dax
Audrid Dax is the fourth host of the Dax symbiont. During her lifetime, she rises to become the head of the Trill Symbiosis Commission and gives birth to at least two children, Neema and Gran.