Lord Jim is all about storytelling. There are stories within stories within stories and then some. As characters tell their stories to our narrator, Marlow, they struggle to find the right words to say just what they mean, and what they have witnessed. Marlow, on the other hand, never seems at a loss for words as he retells their stories to his audience. Perhaps he's such a good storyteller because he is an excellent listener. The dude remembers everything, and everything comes back to our protagonist, Jim. We readers put each individual story that Marlow hears into the larger story of Jim – his rise and fall as a sailor, and his rise and fall on Patusan. By the end of the novel, we've had to do some serious puzzle work, sure, but we also are rewarded with a rich, multilayered story of a complicated protagonist and the ripple effect his life creates on those around him.
Marlow is a classic unreliable narrator. He always puts his own spin on events, and his view of Jim is totally biased. Therefore, it's impossible to find out who Jim really is.
Conrad includes so many stories within stories and jumps around in the narrative to show that it is impossible to get a full picture of a character. We can only know the Jim that others see.