Throughout the story, the narrator is a fundamentally good guy, even if he's got his share of troubles. However, we do see two dramatically different versions of the same character in this story. In its first half, he's our first-person narrator, who addresses us like an old chum; he's a soldier not yet destroyed by the war. In the second half, though, he's Sergeant X, broken down and changed by wartime. Let's examine both of these sides of this fascinating character separately.
In the first half of the story, our narrator is a young soldier just a year out of college, who fosters hopes of becoming a real short story writer. He's a pretty funny dude – he's got a dry sense of humor and a keen, observant eye. He's an enjoyable companion, and, through him, we get the feeling that his experience of the war isn't actually too terrible (yet, that is). That isn't to say that it's great, but still – this isn't the voice of a hardened, traumatized soldier. Instead, it's the voice of a young man who's certainly not happy to be in a war, but isn't to tally miserable, either. He's still able to view the terrible events of the world around him with some levity, and manages to maintain his sense of humor despite his circumstances. As he tells Esmé, he's getting increasingly familiar with "squalor" (read: the war), but it doesn't seem to have gotten to him yet.
The narrator takes an immediate interest in both Esmé and Charles – their youth and the way in which both of them, especially Esmé, handle their family's tragedy is fascinating to him, and to us, his readers. There's a certain tenderness about the narrator that's really appealing; he obviously cares about these kids. His interaction with them reveals a kind and, as Esmé would say, "sensitive" nature. He's the type of guy you might imagine growing up to be a great dad – he knows how to talk to kids in the best possible way, by not talking down to them. We know that he's scheduled to leave the relative safety of Devon the day after we meet him, and we hope that nothing too awful is in store for him…
A year later, the good-natured, comical young man we met in Devon is gone, replaced by a battered, troubled, battle-scarred soldier. The gentle, wry sense of humor we saw him employ with Esmé and Charles is warped into the cruelly sardonic tone he uses at times against Clay. Terrible things have happened to Sergeant X over the last year since his meeting with Esmé, and he's almost unrecognizable as the same character. His severe depression is obvious, and he's recently returned from two weeks in a hospital (where, we learn, he was treated for a nervous breakdown). He's far from "cured" – in fact, his nerves seem to have drawn tauter and tauter as time goes on. Sergeant X's experience of the war has horrifically transformed his whole view of the world, and, even when he tries to pull himself out of his depression, he can't. As the narrator tells us, he's so cunningly disguised that we almost can't recognize him – this is only partly a joke.
While it's obvious that Sergeant X is the narrator from the first half of the story, we are astonished and horrified to see how much he's changed. It seems like he's a totally different person, one who has possibly lost that inherent gentleness and optimism that made him so appealing in the early part of the story. However, at the end of the story, Esmé's letter seems to draw Sergeant X back closer to his old self, our familiar narrator; we get the feeling that maybe he's going to be OK after all.
The end of the story isn't exactly the end of the narrator/Sergeant X – we know from the story's narrative frame (the wedding invitation) that he does in fact survive the war with his f-a-c-u-l-t-i-e-s intact, and goes on to write the very same story we're reading. This is an ultimately optimistic story, full of gratitude and hope; though the war came close to destroying this one man, he was ultimately able to live through it and tell the story. Because of his transformation into the writer, there's some speculation that the narrator/Sergeant X is actually Buddy Glass, the very present narrator/author-figure in a bunch of Salinger's other stories, including "A Perfect Day for Bananafish" and Franny and Zooey. However, this claim is unproven, and there are good reasons both to buy it and not to buy it. Just thought you should know.The Narrator (Sergeant X) Timeline