Odwald Belze is supposedly Pressia's grandfather, though we aren't sure. Either way, he cares for her with all of his heart, and would do anything to protect her. Unfortunately, we don't get to see much of Grandpappy because Pressia has to run away from OSR early on in the book. Either way, the thing that sticks out the most about her grandfather is how he was fused from the Detonations:
He'd been clutching a small handheld electric fan when the Detonations hit—it happened during summer—and now the fan is with him forever. Sometimes he labors to breathe. The spinning mechanism gets gummed with ash and spittle. It will kill him one day, the ash mounting in his lungs, the fan chugging to a halt. (1.38)
This fan becomes an important object to remember as the story goes on, especially because the fan is what Ellery puts in the little blue box to show Pressia that her grandfather is deader than a doornail.