Prior to the events of Jedi Knight
Sariss was a Priestess of the Dark Side who left her church to join Jerec in his mission to find the Valley of the Jedi, a source of nearly unlimited power. To help supply Jerec's search with funding, Sariss seduced a wealthy baron who owned a ring of mineral mines and was engaged to marry him. The baron's son, Yun, also falls in love with her. She realizes that Yun can use the Force and begins to teach him how to control it.
Sariss tells Yun that she also loves him, but that they cannot be together because of her engagement to his father. She convinces Yun to murder his father and then gives the baron's wealth to Jerec, though her mission had yielded an even more valuable reward: a new Dark Jedi to join their cause.
During the events of Jedi Knight
The search for the Valley of the Jedi eventually leads them to a Jedi named Qu Rahn. As shown in the opening movie sequence, the Dark Jedi find and capture him, but he is unwilling to divulge any information of the Valley's location. Jerec then uses the Force to tear the information from the Jedi's mind, learning that Rahn's friend Morgan Katarn (whom Jerec had murdered years prior) had known the Valley's location and kept a map hidden in his home on the planet Sulon.
Qu Rahn, knowing that his secret had been discovered, used the Force to take Yun's lightsaber and attack the Dark Jedi. The aging Jedi manages to fight off each of Jerec's followers in turn, even managing to sever Maw's legs in the process, before Jerec steps in and kills him.
With the aid of the droid, 8t88, they manage to decipher the map hidden in Morgan Katarn's home and learn the location of the Valley of the Jedi.
You first run into Sariss in game after the battle against Maw. She and the other Dark Jedi approach Kyle Katarn (son of Morgan and the character you play as in the game) and Jerec congratulates him for killing Maw. He then tries to get Kyle to fall to the Dark Side and kill Jan Ors and join him. Depending on whether you have chosen the Light Side or the Dark Side, Kyle will either refuse outright (Light Side) or kill Jan and refuse to join Jerec, wanting the power of the Valley for himself (Dark Side). Regardless of what Kyle does here, Jerec blasts him back onto his ship and sends the ship crashing towards the planet's surface.
If you have chosen the Light Side, Kyle is later captured by Sariss, Boc, and Yun. Boc smashes the Jedi's lightsaber and Sariss attempts to execute Kyle, but Yun throws himself in the way of her attack. Stunned, she asks him why he has sacrificed his life to save Kyle. He r esponds by saying "He's a Jedi. He deserves a battle." Honoring Yun's final wish, Sariss frees Kyle (who takes up yun's lightsaber) and duels him herself.
Sariss can be a particularly challenging fight due to her use of the "Deadly Sight" Force ability, which allowed her to deal heavy damage to everyone in her field of vision. She is the only user of Deadly Sight in the game, and in fact, in the entire Star Wars universe (though you can acquire it in game, if you choose the Dark Side).
If you have chosen the Dark Side, Kyle instead faces and kills Yun before going on after Jerec. Sariss abandons Jerec and joins Kyle Katarn if you choose the Dark Side.