Seifer (symbiont)

Seifer was a genetically altered Trill symbiont, a symbiotic life-form that existed with the Lex host and then the Night host as a joined species. Each joining of Seifer with a host created a new, unique individual, but each individual also carried the memories of the previous hosts. 

Genetic mutation
In 2379, the Gordarions kidnapped Commander Seifer and experimented on the Seifer symbiont. Removed from the host, the symbiont gained a mouth and a minor suggestion of appendages. When the crew of the USS Phoenix-X saved both the host and symbiont from Gordarion captivity, the Seifer symbiont began giving orders to the crew. In no time, the symbiont had taken its place upon the Captain's chair and attempted to command the ship during the Gordarion crisis. Doctor Lox then re-aquired the symbiont and began surgery. The symbiont was soon returned to the Night host. ("Occurrence, Part I & II")

Mirror universe
Because the symbiont's genetic alterations took place in close proximity to a Rift that radiated quantum disturbances, the symbiont's ability to become self-conscious became only possible under abnormal quantum circumstances. This was not limited to the Mirror universe.

In 2382, when the Phoenix-X was brought into the Mirror universe, the Seifer symbiont became self-sustaining once again, and literally burst out of the Night host. After an escape, and a quick mending to the Night host by Doctor Lox, the Seifer symbiont was free yet again run a muck. This time, the Seifer symbiont used the crew's at-the-time-capture to his advantage and attempted to sell the crew to the Klingon-Cardassian Alliance in exchange for his own command. His efforts were quickly thwarted by the crew, and the symbiont was returned to the Night host after the return to the original universe. ("Mirrorlyness!")

Personality
The Seifer symbiont is constantly obsessed with power. It would attempt to gain it any way possible, even if the means were immoral or illegal. This closeness to negating the law is somewhat evident during the joining with its hosts, as both the Lex host and the Night host regularly didn't hesitate to work against the law. ("Secret Shuttles, Part I")

Background information

 * During the episode "Occurence, Part II" the symbiont's name was written as -Symbiont-, and during the episode "Mirrorlyness!" it was simply Sym. In both cases, the name should have technically been Seifer, but Seifer was used as the Night host's name - despite the fact that the Seifer symbiont was seperated from the Night host.