In many ways, Marion Peters is an extremely unpleasant character. She's uptight, she's humorless, and she's standing in the way of our protagonist. On the other hand, just as was the case with Charlie, Fitzgerald doesn't let us off the hook by making her an entirely villainous character. She has a point, for instance, when she gives Charlie a hard time for running to the Ritz bar first thing when he got to Paris, and she makes it in a subtle enough way: "I should think you'd have had enough of bars" (1.45).
Since we've seen that Charlie may not be the new man he claims to be, we might agree with Marion that Honoria is better off staying in the stable, loving household of her aunt and uncle. And yet, we can't totally agree with Marion, because she seems driven not by what's best for Honoria, but by sheer hatred for Charlie. Consider her outburst in the story's third section:
"Do what you like!" she cried, springing up from her chair. "She's your child. I'm not the person to stand in your way. I think if it were my child I'd rather see her – " She managed to check herself. (3.48)
Whoa there – does anyone else think that Marion was about to say that she'd rather see Honoria dead than with Charlie? It's lines like this one that shift the reader toward Charlie's side and away from Marion's.
Marion is also an important character for the story because it is only in describing her that the narrator breaks from Charlie and becomes, for a few select passages, truly omniscient. Check out "Narrator Point of View" for more on the consequences in form. But here, take a look at the content of these important passages:
With each remark the force of her dislike became more and more apparent. She had built up all her fear of life into one wall and faced it toward him. (3.35)
Marion shuddered suddenly; part of her saw that Charlie's feet were planted on the earth now, and her own maternal feeling recognized the naturalness of his desire; but she had lived for a long time with a prejudice – a prejudice founded on a curious disbelief in her sister's happiness, and which, in the shock of one terrible night, had turned to hatred for him. It had all happened at a point in her life where the discouragement of ill health and adverse circumstances made it necessary for her to believe in tangible villainy and a tangible villain. (3.41)
Now we're leaning in Charlie's direction again; it seems unreasonable of Marion to make a symbol of Charlie, to blame him for the faults of the lavish extravagance she despises in an entire group of people. We start to feel that her attitude towards him just isn't fair. Why should Charlie take the fall for the faults of his entire generation? And yet, isn't this what Fitzgerald asks of his character as well? To represent the wasteful extravagance and too-late regret of a generation of Jazz Age drinkers and party-goers?