Antagonist
Character Role Analysis
Fate
Plenty of people are responsible for the bad stuff that happens to our main characters in the book, no one or nothing more so than fate itself. While you could argue that Jacob gets in his own way of happiness sometimes, or that the war damages Jacob and Geertrui, you could easily poke holes in these arguments. After all, it's not the war but a heart attack that kills Jacob (Geertrui's main man) in the end—and Jacob (our main man) is learning to define himself when the book begins.
But fate—or simply bad luck—gets in the way of both of our protagonists. Geertrui herself acknowledges this when the war is going on. She tells us:
We were lucky that the German had not spotted us before he stood up, we were lucky that he hesitated, we were lucky that Dirk moved so quickly, we were lucky that Jacob's gun was ready to fire, and we were lucky that the gun's mechanism worked properly despite the conditions. As so often at such times, especially in war, the outcome depended on luck. Not on heroism, if heroism depends on rational thought, for there was no time for thought. Only on the irrational, arbitrary, unjust nature of luck. (10.3)
Earlier Geertrui thinks Jacob is crazy for saying he was hurt only because of bad luck, but by the time she's lived through more of the war, she thinks the same exact thing. Her comments show us that war is just as much about luck as it is about courage—and sadly for the lovers, sometimes your luck runs out.