Jadvyga is a lot like Ona: she is an honest working girl who must support her large family with her wages. Also like Ona, she is unlucky. She is engaged to a man named Mikolas, but he has been struggling with work-related injuries (an infected hand, much like Marija Berczynskas). She cannot marry Mikolas until they are more financially secure, and they are never financially secure. By Chapter 15, their engagement is officially over. Mikolas has lost his hand to infection, and her family has decided Jadvyga can't afford to marry him.
Jadvyga's main plot importance (besides providing yet another example of the horrors of Packingtown life) is that she is Ona's alibi. When Jurgis asks Ona where she has been after a night out, Ona claims that the cold weather forced her to stay the night at Jadvyga's apartment rather than walking all the way home. When a second night passes without Ona coming home, Jurgis looks for her at Jadvyga's. It turns out Ona has never been to Jadvyga's. When Jurgis accuses Ona of lying, the whole story of her rape by Connor comes out. Ona has been trying to hide it to protect Jurgis from the knowledge that he has failed to protect his wife. She knows that, once Jurgis finds out what Connor has done to her, he will try to kill Connor and ruin the family. So Ona uses Jadvyga to hide what is really going on for as long as she can – roughly two months. It all comes out in the end, of course.