Sarty and his Brother
On several occasions Sarty thinks people are talking to his brother when they are talking to him. This observation is a foil alert. The brother, Abner's active barn burning accomplice, is a snapshot of what Sarty would likely become if he stayed with his father and been converted to his way of life. We realize that Abner is likely training Sarty in the same way he trained his brother before him. The brother's presence in the story heightens our sense of relief when Sarty runs away.