Seska was a 24th century Cardassian operative, surgically altered to appear Bajoran in order to infiltrate the Maquis. Before being stranded in the Delta Quadrant with the rest of her Maquis team and the USS Voyager crew, she was assigned to Chakotay's Maquis raider, the Val Jean and used her relationship with him to steal Maquis secrets.
Her true origins remained undiscovered until the time she helped transfer replicator technology to the Nistrim and was seriously injured during the transfer. The Doctor's subsequent examination caused her true Cardassian nature to be revealed. She denied this fact, claiming that she had suffered from Orkett's disease as a child and received an extensive transfusion from a Cardassian to survive. When Chakotay confronted her, revealing the cover-up, she finally confessed to everything and openly expressed her dislike for Captain Janeway's decision, calling her a fool for having gotten them stranded in the Delta Quadrant and unwilling to give the Kazon what they wanted so Voyager had an ally. She also expressed her disappointment in Chakotay, whom she believed had naively followed Janeway's footsteps, and stated that she couldn't imagine why she had ever loved him. Before she could be further questioned, she managed to escape Voyager and transferred aboard a Kazon raider.
After Seska left Voyager, she immediately restored her Cardassian physiology and became the consort of First Maje Culluh, the leader of the Kazon-Nistrim sect. Culluh had a thirst for power and she used that to manipulate him to do what she wanted. Equipped with Starfleet, Maquis, and Cardassian tactical experience, she provided the Nistrim with expertise on Voyager and schemed ever newer ways of stealing the ship and its technology.
In late 2372, Seska gave birth to the child she claimed to be Chakotay's. She immediately contacted him, claiming that Culluh had discovered that the child was not his and threatened to have the child be raised as a slave. This lured Voyager into a trap in which a Kazon operative named Tierna was able to enter the ship and detonate organic explosives in his bloodstream while on board, disabling the ship's power systems and the primary computer core. Janeway was unable to activate the auto-destruct sequence in time and the ship was boarded by Kazon troops. Soon after Seska and Culluh took over the ship, they marooned its crew on a desolate planet named Hanon IV.
During the Kazon occupation of the ship, Seska took stringent precautions against any possible threats, even questioning The Doctor's loyalty. That line of questioning was cut short when he revealed through a DNA analysis that her baby was actually half-Kazon, not Human, and thus not Chakotay's. This greatly disappointed Seska, who had been sure that the baby was half-Human and was hoping to forever trap Chakotay by her side.
After taking over the ship, Seska and Culluh did not get very far, however, as most of the Kazon were ill-equipped to navigate and control the ship's complex and unfamiliar technology. In addition, Tom Paris - who had left Voyager in a shuttle prior to the Kazon attack to seek help - with assistance from Talaxian forces, as well as The Doctor and Lon Suder, was able to retake the ship. During the mission to recapture Voyager, Seska was mortally wounded in the resulting overload of the console she was at and only had a few minutes to live before she collapsed with her baby in her arms. Culluh discovered her body and escaped with their son just before the crew returned.