Marion is an antagonist in the sense that she opposes our protagonist. She's the main barrier standing in the way of Charlie getting his daughter back (which, we know from his "Character Analysis," is symbolic of a larger attempt to get his life back on track). She's also unlikable at several points in the story.
On the other hand, what makes "Babylon Revisited" so brilliant is that Fitzgerald doesn't let us just hate Marion. As much as we'd like to condemn her for being uptight, she sometimes has a point (as when she gives Charlie a hard time about going straight to the Ritz bar the moment he got back to Paris).
As we discuss in Charlie's "Character Analysis" and "What's Up With the Ending?", Charlie is in many ways his own worst enemy. Like it or not, we have to face the fact that Charlie sowed the seeds of his own destruction when he left his brother-in-law's address for Duncan at the very start of the story. We sense that he still has that self-destructive impulse, and we can't ignore the moments when he seems to long (however subtly) for his past life. He's also still drinking, which is generally not a good idea for a recovering alcoholic.
Duncan Schaeffer and Lorraine Quarrles
Duncan and Lorraine definitely ruin everything for protagonist Charlie Wales, which means they are also good candidates for antagonist. As we discuss in their "Character Analysis," they basically pursue him through the story, and not in a pleasant way, either. There's a great line in which Fitzgerald even makes them parasitic, as though they want to feed off Charlie.