Sure, they live in totally different countries and wouldn't do well on a blind date together, but we're including Maria and Philip in the same character analysis because they play off each other so well.
Maria is a solider in the Russian military. Her platoon is forced to engage the zombies without being told what they are fighting. This lack of information causes the platoon members to distrust their higher-ups and attempt a revolt. Spetznaz soldiers nip that in the bud, and, as punishment, the platoon has to kill one-tenth of their numbers. With rocks.
Philip has his own problems with military orders. While guarding civilians in Hamburg, he receives the order to retreat and is told to leave the civilians behind. Outraged, he follows the orders anyway, unaware that he's become part of Germany's version of the Redeker Plan.
Both Maria's and Philip's stories have a similar outline. The soldiers engage the zombies, are presented with commands they do not agree with, fight against the command, eventually succumb to it, and pay the price for following orders. But they way they deal with their circumstances is wildly different.
Maria is ordered to kill her friend Baburin and she does so. In this act, she "relinquished [her] freedom that day, and [was] happy to see it go" in the name of just "following orders" (4.4.31). When we see her later in the "Good-byes" chapter, she has joined a stable of women who serve Russia via their uteruses (i.e. state-run pregnancy program). She has given up her personal freedom and individuality so as not to claim responsibility for any of her past actions.
Philip, on the other hand, refuses to let anyone else take the blame but him. As he notes:
You can't blame anyone else, not the plan's architect, not your commanding officer, no one but yourself. You have to make your own choices and live every agonizing day with the consequences of those choices. (5.2.25)
These two characters continue to strengthen the novel's emphasis on individuality and self-reliance. Maria attempts to shrug off her individuality and ends up in a literal prison, her ability to make decisions revoked by the Russian state. Philip's decision to "shoulder the weight" might have been the painful one, but he's at least out and living in the world (5.2.25). Although burdened by the past, he has the ability to choose his own future.