Symbolism, Imagery, Allegory
While the Window Woman is technically a person, since no one ever sees her or is even sure that she actually exists, we're going to treat her as a symbol. And while nobody likes to be treated like a tool (and a symbol is most definitely a literary tool), we think she'd be pleased with what we think she represents: beauty and feminine power.
Parvana isn't thrilled about having to sit in the marketplace, which is pretty legit since her family's dressed her up as a boy and sent her out to more or less save them from complete starvation, so she sits there on her blanket and looks to the sky, wishing Father would return. And that's when she notices a "flicker of movement" (7.48) coming from the blacked out window. Was she imagining it? It doesn't matter—just the idea that someone is watching over her gives her hope and comfort.
And then Parvana sees the flicker again, except this time it's a woman's face, and she gives Parvana a quick smile before she pulls the window shut. Remember: women are not to be seen, not even in windows, so this is a big risk that the woman takes. Then a few days later, presents start falling on Parvana's blanket—a white handkerchief, a perfect red bead, and an embroidered cloth. These little gifts are signs of beauty in the ugly face of war, treats for the child down below the woman's window who has been forced into a very adult situation.
Needless to say, the Window Woman's gifts make Parvana happy. They are small bits of kindness and beauty in a challenging time, and make Parvana feel less alone. And insofar as these gifts and Parvana's sense of the woman's presence impact her experience in the marketplace, the Window Woman represents the fact that despite the Taliban's oppressive rules for women, they still have power; women are still present even as the Taliban tries hard to make that not true. And that, of course, is pretty beautiful in its own right.