Fans of Star Trek: Deep Space Nine will be familiar with the Dax symbiont, a wormlike entity that joins with a race known at the Trill. After each host dies, the symbiont passes to a new host, who retains the memories of the prior hosts. The Dax symbiont has been alive for over 300 years and had been bonded with four male hosts and five female hosts (as of the final season.)
In the first season of DS9, the current host was a Trill chief science officer called Jadzia (Terry Farrell).
Jadzia was kind, confident, pragmatic, with the heart and spirit of a warrior. She was happy to be a host to the symbiont. Her dearest friend was the station commander Benjamin Sisko (Avery Brooks,) who had first met Dax during its Curzon host. The Deep Space Nine doctor, Julian Bashir (Alexander Siddig) had a close friendship with Jadzia and harboured romantic feelings for her. Over the years he moved on, especially after Jadzia began dating and eventually married the Klingon Worf (Michael Dorn). During her time in Deep Space Nine, Jadzia was involved with many away missions and during the early days of the Dominion Wars, she was a commanding officer aboard the USS Defiant.
Terry Farrell chose not to renew her contract during the six season, and as a result Jadzia was killed off in the season finale, "Tears of the Prophets." She was injured by Gul Dukat, and Bashir was able to save the Dax symbiont, but not Jadzia, who lived long enough to see her husband one last time. Jadzia and Worf had previously decided to try to have a baby despite the complications of a Trill/Klingon pregnancy.
Ezri Tigan (Nicole deBoer) was a Trill Starfleet officer aboard the USS Destiny, where she served as an assistant counselor. Their mission was to escort the Dax Symbiont back to Trill so that it could be joined with its next host. Things took a turn for the worst and the symbiont needed an immediate transfer to a new host. Ezri volunteered despite not desiring to be a host and thus having not undergone any of the required preparatory training. When the surgery was complete, she spent time at the Symbiosis Institute on Trill, learning to adjust to the onslaught of new memories and feelings.
Ezri Dax transferred to Deep Space Nine, where she was promoted to the station's counselor despite not having finished her training. Like Jadzia, Ezri was kind and strong, but she was far less confident. She began very timid and unsure of herself, but as then season progressed, she became more confident and able-bodied. She retained her feelings for both Worf and Bashir, but the personality difference led to her choosing Bashir to be with. Worf was still very much in love with Jadzia and was unable to compare the two women, especially since Ezri had all of Jadzia's memories.
DS9 will never be my favorite Star Trek series; the series on the whole was rather dull in my opinion, but it did have a very strong, emotional finale. The characters—to me—seemed rather one dimensional and lacking, with the exception of the Ferengi Quark and his brother Rom. However, my favorite character was Jadzia. Every DS9 personality quiz has lauded me as her. When I learned that she was being killed off I was very upset, but I was determined to finish the series regardless. Even though I enjoyed Nicole deBoer as an actress in The Dead Zone, I was adamant that I could never like her character as much as Terry Farrel's. Fan comments on YouTube did nothing to change my mind on that assumption. Several people referred to Ezri as weak and whiny, two of my least favorite personality traits. She was also called "a replacement Dax" and "ill-conceived."
By the end of Ezri's first episode, "Image in the Sand," I was absolutely in love with the character. She was more relatable to me, as someone who is very nervous and diffident, I thought her evolution from scared little girl to bad ass woman was a real treat. I have come to like Ezri just as much as—if not more than—Jadzia. The two characters were really the saving grace for me. DS9 was a lot darker and more claustrophobic than I'm used to, especially in Star Trek. Personally, I feel that the series would have benefited from a shorter length. It was not interesting enough to keep my attention for seven seasons.
I loved the whole Dax/Trill symbiont relationship. I can't imagine waking up one day with a worm inside your body, and suddenly you're invaded by memories and feelings from other people. How would one know if their feelings toward someone were their true feelings, or the feelings of a past host? I thought it was exceptionally brilliant to have Ezri be an unwilling host so that we could see another angle of the host role. Jadzia had happily taken on the role and had trained in it, whereas Ezri had just wanted a normal life as a Starfleet counselor. Still, over time, Ezri accepted her burden and became a lot stronger. Though, one could question whether the strength came from within her, or from the Dax symbiont and the previous hosts... interesting thought.