Pilar is the gypsy mujer of Pablo. She has been with him for several years, and one of the purposes she serves in the book is to give the reader background information. It is through her that we see the beginning of "the movement," the way life was before the war, and the courage of Pablo before he "went bad." Pilar also plays a major part, through references to her reading of Jordan's palm, in maintaining the feeling of impending doom.
But even these purposes are not enough for Hemingway. As mentioned above in relation to Pablo, Hemingway does not let his major idea be represented by one character alone. In the book we see Robert Jordan in transition from one concept to another. Pablo, too, is on that same road, a few miles, as it were, ahead of Jordan. And now we find Pilar traveling the same road, but "bringing up the rear." It is she who supplies the allegiance to the cause which is dead in Pablo and dying in Jordan. "I am for the Republic," she says in her showdown with Pablo, "and the Republic is the bridge." She is late, too, in her realization of the real danger involved in the bridge assignment. And it is only in the final pages of the book that she reaches that stage of her transition in which she begins to doubt the value of the cause.
One of the major ideas of the book, then, is the transition, from one belief to another, of the three characters described above. By placing each of these characters at a different stage of development, Hemingway has enabled himself to portray the three stages of this transition and yet retain the emotional intensity provided by the brief time span covered by the book.