She was played by Sean Young, who is perhaps best known for playing Lois Einhorn in Ace Ventura: Pet Detective.
Jennifer Kamplan is the recently promoted head of an advertising company. She hires a new assistant, Stacy Sherman, who possesses a hidden sinister side, as it turns out she has been showing a deranged devotion to Jennifer by killing her opposition. Stacy's obsession even leads to her breaking into Jennifer's house and putting a knife into Jennifer's husband, Patrick. Jennifer rushes to home to see Stacy, who tells her that she has killed her husband. She later pulls out a gun from her drawer and shoots Stacy on her shoulder.
At that moment, Jennifer reveals her true game plan; her marriage was falling apart and Patrick wanted out, so Stacy killing him is the perfect way to end the marriage without having to give up half of what she has in a divorce settlement. She also told Stacy about her devotion to her and how she's using it to get what she wants, while stating that Stacy has always needed her. After her speech, the evil Jennifer shoots Stacy again, this time killing her and cementing Jennifer's heel turn. She later deshelvels herself and tells police that Stacy killed her husband and came after her, which is why she shot her. At the very end, however, it turns out that Patrick survived after they find a pulse, and the last scene shows Jennifer with a worried expression on her face, as her evil plan is falling apart.