She later follows her husband up a staircase and stabs him with a screwdriver, killing him, and then presumably kills herself. She is seen lying dead of a stab wound.
When Sue sees into the past, the Valley Girls repeatedly ask, "Who is she?" in regards to this woman. Later, when Doris realizes that her husband had an affair with Sue, a flash of the dead "Who Is She?" woman appears, linking her to Doris.
David Lynch seems to have made a decision not to give this character a firm identity. In the scene where she stalks her husband up a staircase, she seems to be played by Karolina Gruszka, who plays the Lost Girl. When she is seen dead, she seems to be played by Julia Ormond, who plays Doris Side, the modern-day reincarnation of the role.
A mysterious woman is also seen looking at Chelsi when the Valley Girls briefly transport to old world Poland while Doris is stalking Sue in LA. This may be the "Who Is She?" woman.