Seska was one of the crew of Chakotay's Maquis ship when it was pulled into the Delta Quadrant by the Caretaker's array. Early in the show, she appeared as only a minor member of the crew, but there were hints that there might have been some closer relations between her and Chakotay.
Delta Quadrant Treachery
After some Federation technology was found on board a Kazon ship, an investigation lead to her being discovered to be a Cardassian secret agent. She quickly arranged for herself to be beamed to a Kazon ship and joined them. She arranged to capture Chakotay, and while she had him in her custody, she took a sample of his DNA and used it to impregnate herself. This was a set up to draw the Voyager into a trap.
For a time, she used an inside operative on board Voyager to gain important information on the ship's course and activities. She also used this operative to slow and divert efforts by the ship to distance themselves from Kazon space. Until the operative was forced into the open by the bumbling and ineffectual investigative efforts of Neelix (who originally accused Tom Paris, ironically the guy actually doing the investigation job right, of being the spy until set right by Tuvok), he was very effective at sabotaging the ship from the inside.
The opening move of the plot to capture Voyager included infiltrating a Kazon onto the ship with an explosive compound hidden in his blood. Using the hatred of Voyager created by Janeway's botched attempt to broker peace between the Kazon and the Trabe, Seska was able to convince the various Kazon clans to cause specific damage to Voyager's Auxiliary Command Processor, making the use of self-destruct impossible. The Kazon operative then detonated himself to help disable the ship, allowing the Kazon to board and capture it. They marooned the crew on a desolate planet to fend for themselves. This led to the deaths of at least two crewmen because of Neelix's stupidity.
Seska died from exploding console disease during the attack on Voyager by a combined fleet of Talaxians and Tom Paris that took back Voyager. After the ship was retaken, the Doctor examined the child that was supposed to be Chakotay's and found that one of the Kazon leaders was the real father.