While he's not really anyone's grandfather biologically speaking, Grandfather is nonetheless the patriarch of the group of slaves who cross paths every day at the Tea Water Pump. Clearly, he's the oldest of the bunch, "with stone-gray hair and skin the color of the night sky" (13.29). The thing that endears him most to Isabel, though, is that he has a scar on his cheek traditionally given to young boys in Africa when they become men. Isabel's father had the same mark, and as a result, Grandfather provides her with a connection to the home she's lost.
Grandfather only makes a couple of appearances in the book, but he gives Isabel the key piece of advice that ends up being her guiding light on the journey to freedom. "You must choose your own side," he says to her of the war, "find your road through the valley of darkness that will lead you to the river Jordan […] Everything that stands between you and freedom is the river Jordan" (26.44, 46). His words provide the catalyst that starts Isabel thinking beyond the concrete terms of Patriot or Loyalist and toward the process of making her own way.