Tropic of Cancer
If Henry is the protagonist, just about everything else is the antagonist: women, his "friends," modern living, work, and especially America. All of these things are just getting in his way.
Miller and Moldorf
Though we think Van Norden is pretty foily, Miller himself describes Moldorf as his double: "We have so many points in common that it is like looking at myself in a cracked mirror" (1.31). That's pretty much the definition of foil right there.