Tools of Characterization
This tool applies most directly to Marion Peters. Most of what we know about her comes from explicitly telling passages like these:
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)
There's not a lot of explanation needed here, because Fitzgerald explicitly tells you how it is with Marion.
On the other hand, you might read some trickery into these passages if you're asking just how close the third person narrator is to Charlie at this point. Are these the words of an objective narrator, or the thoughts of Charlie, indirectly presented?
The most obvious name to discuss is that of Charlie's daughter, Honoria. Because of the echo of the word "honor," we see that Charlie's attempts to get his daughter back might be symbolic of a greater struggle on his part to restore the honor he lost during his destructive years in Paris. Honoria is the goal of his trip to Paris, but honor is the goal of his reformation.