What role does Simon play on the island in Lord of the Flies?I know he plays many figurative roles and holds a lot of symbolism in the story. But...

Simon is the mystic, or spiritual leader.  Simon goes into
trances due to an unnamed medical condition.  The description fits petit mal seizures,
but the book doesn't elaborate.  Simon is the boy who is always kind to the other boys. 
He is a worker who does not complain.  He seems to know and understand concepts before
the others do.  In chapter 3, when Ralph is complaining that the boys aren't doing their
fair share of the work because they are off playing, it is pointed out to Ralph that
Simon is always there working and helping.  Then at the end of that chapter, we see
Simon going off by himself and hiding under the creepers and bushes.  "Holding his
breath he cocked a critical ear at the sounds of the island."  Then in chapter 5, at one
of the assemblies that Ralph has called, when the littluns are afraid there is a beast
on the island, Simon says maybe what they should fear is themselves, "What I mean
is.....maybe it's only us." At the end of chapter 8, it is Simon who has the
conversation with the Lord of the Flies when the Lord of the Flies tells Simon that
indeed, the source of evil on the island is the boys themselves.  The conversation is
only in Simon's head, so what is actually happening is Simon is coming to full
understanding of what is going on.  He is finally able to articulate what is happening
to the boys on the island as their society is disintegrating around them.  No one else,
at this point, clearly understands what is happening - only
Simon.