For the most part, the novel focuses on industrialists and businesspeople. But artists are also a part of Galt's strike, as we discover in Atlantis. Art is an important part of Galt's philosophy, since it's a type of work that expresses a person's values.
Kay Ludlow is one of the few artists profiled in great detail. She left the real world for artistic as well as gender reasons:
"Whatever quality of human greatness I have the talent to portray – that was the quality the outer world sought to degrade. They let me play nothing but symbols of depravity, nothing but harlots, dissipation-chasers, and home-wreckers, always to be beaten at the end by the little girl next door, personifying the virtue of mediocrity." (188.8.131.52)
In Atlantis, Kay is able to be the type of artist she wants to be.
Kay's other notable role is that of Ragnar's wife. What would lead her to marry a pirate who is gone all the time? Perhaps it was some biological imperative to produce absurdly good-looking, Nordic uber-children. Actually, though, Ragnar and Kay have something in common: both are more creative, life-of-the-mind types than the other industrialists. Ragnar did want to be a philosopher, after all. Their final scene together demonstrates this:
Kay Ludlow sat before a mirror, thoughtfully studying the shades of film make-up spread open in a battered case. Ragnar Danneskjöld lay stretched on a couch, reading a volume of the works of Aristotle. (184.108.40.206)
Kay's other role is as the only other woman in Akston's little circle, comprised of his three sons and their spouses essentially. This allows her to serve as a female companion to Dagny. In terms of her morals and brains, Dagny is miles beyond the book's other real-world women. Kay makes a nice counterpart to Dagny, representing the humanities and arts side of Galt's values, while Dagny takes care of the business side. Both of them are ideal women who live by values similar to Galt, so it makes a lot of sense that Kay would be married to the third member of Galt's inner circle.