North and South
Sure Henry and Charles are foils. Quentin and Shreve, too. But we'd say the most stand-out foil is the North and its relationship to the South. Because this is a novel primarily about the South, Shreve's presence and Quentin's time at Harvard provide a stark contrast: instead of instinctual Southerners, we are exposed to intellectual Northerners. And since they're the ones who survive, it seems like Faulkner is celebrating the triumph of reason over instinct. Pretty deep for a set of foils, don't you think?