What does Arch represent in Kneel to the Rising Sun?

Arch Gunnard, the plantation owner, represents a system that exploits both African Americans and poor whites. It's not in Lonnie's interests to support the system represented by the cruel, oppressive Gunnard, but he does so anyway, initially through inaction, then later on, more actively. He feels deeply resentful and ashamed at having to beg Gunnard for rations for himself and his starving family, yet he does not challenge the existing order.
Lonnie is representative of many poor whites laboring under the lash of an exploitative agrarian economy. He has absolutely nothing in life, but he still has one distinct advantage over African American sharecroppers like Clem Henry: his race. When push comes to shove, Lonnie sides with Gunnard, even when it's obvious that Gunnard's cruelty is directly responsible for his father's death. He physically attacks the innocent Clem and helps Gunnard hunt him down as he hides in the woods. Instead of showing solidarity with a fellow worker, another victim of oppression, he joins forces with his oppressor to hunt down an innocent man.
The metaphor is inescapable. Those in positions of economic and political power often resort to racism as a classic divide and rule tactic to weaken their subordinates, thus consolidating their domination.