"Babylon Revisited" is narrated in a close third person, meaning that we only see things through Charlie's eyes, and are privy to his thoughts and observations. Normally, you might hear that the close third person brings the reader closer to the protagonist and helps garner sympathy for his or her character.
Sure, this is true in "Babylon Revisited," but Fitzgerald brilliantly allows for some important space between the narrator and Charlie. What we mean is, the narrator – and the reader – know things that Charlie does not. Most importantly, they know one big thing: that Charlie left his address for Duncan Schaeffer at the beginning of the text, and forgot about it somewhere between the Ritz and the Peters' house. You could argue that this one detail opens up the stage for Charlie's tragic loss of Honoria at the end of the story. Charlie doesn't remember this detail; he's left in confusion as to just how Duncan "ferreted out the Peters' address" (4.26). (We talk about why Charlie doesn't remember in his "Character Analysis," which you should read for more information on this complication.)
The other oddity in the narration comes in the descriptions of Marion's character. Check out these two paragraphs:
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)
You might be able to argue that this first passage is from Charlie's perspective, but the second is clearly not. This is a rather odd decision on Fitzgerald's part, because changing the narrative technique in the middle of the text for such a short amount of time jars with the rest of the story.