Petrus to David
Petrus and David are both strikingly alike and markedly different, but it is both their similarities and differences that make them foils to one another. By observing their interactions, we learn that David and Petrus are both alpha males who seem to be jockeying for position against one another. At least, David seems to feel displaced; as far as we know. We tend to get the impression that Petrus could care less. David represents the alpha male coming from the city, full of knowledge and charisma but lacking the ability or power to get by out in the country. Petrus is his rural counterpart, one who may lack education cosmopolitan experience, but who slowly gains power out in Salem until he becomes a formidable landowner.
Another interesting way of posing Petrus and David against one another is to think about their relationships with and attitudes towards women. Early in the novel, Melanie asks David if he is "collecting her," in a surprised response to his assertion that he doesn't "collect women" (4.11-12). Well, David doesn't collect women insofar as he doesn't have multiple simultaneous long-term sexual relationships, but just by looking at the way he sleeps with so many women, you might say that he in fact does. Petrus unabashedly collects women – we know that he either has two wives or one wife and a serious girlfriend, and by the end of the novel it's apparent that he's going to bring Lucy into the mix as well. Doesn't it seem like David and Petrus just take two different approaches to the same action? The more we learn about Petrus, the more we learn about David through comparison and contrast, and that makes Petrus his foil.
Lucy to David
Sometimes it's hard for us to believe that Lucy and David spring from the same gene pool, and guess what – it's hard for them to believe it, too. When we originally meet David, we don't automatically go, "hey, that swanky university professor from the city – I bet he brought up a daughter who's really into organic farming and keeping dogs out in the country." Then we meet Lucy, and we find that the differences between her and David are not only striking, but also quite revealing. Lucy acts as a foil to David in the sense that her country ways – and her awareness of David's slight aversion to them – don't just give us more information about who she is, but also about who David is. For example, when Lucy tells David that he acts like everything she does is part of his life story and not her own (22. 29), or when she tells David that she knows her choice of lifestyle has disappointed him, she develops an entirely new facet of David's personality that we wouldn't know if we didn't know about the differences between their two characters.