You can’t think of the authors of this story as writing for modern-day readers who expect everything in a story to be a symbol for something. These were not Metaphysical poets. The story has to be read as one would read a folk tale explaining how the elephant got its long trunk. These authors were trying to explain how the idea of “eating at the tree of the gods’ own knowledge” occurred to Adam and Eve. They might have said that it just popped into their heads, but this would not make a very interesting story. So they introduced a character to put it into their heads. Naturally they chose a creature to play this character that is frightening and dangerous; they wouldn’t have chosen a sheep or a sparrow. The serpent doesn’t “represent” anything more than a good choice from a story-teller’s point of view.