Simon is a sort of religious mystic, a prophet, and a figure for Christ in the novel. He is the only boy who realises, instinctively, that the beast is "only us", that it is the "darkness of man's heart" that the boys are afraid of and which constitutes the "beast". It is as he comes down the mountain to bring the good news to his friends that it is only the parachutist, and not the beast, on the hill, that he is killed by them.
Simon constantly has a close identification with nature, going to his special place, surrounded by candle buds, on a couple of occasions: and being unafraid to travel across the island alone to help Piggy with the littluns. Simon isn't afraid.
Even when everyone is frightened of the parachutist on the mountain, Simon volunteers to go up the mountain: "What else is there to do?" he asks.
Yet Simon's clearsightedness and insight is juxtaposed with his physical weakness, and epilepsy. He is always throwing faints, Jack tells us early on, and after his encounter - in his imagination - with the Lord of the Flies, he loses consciousness and falls into the imaginary mouth of the beast.
Simon has the insight, the fundamental understanding, that would save the boys on the island. Yet he never manages to communicate it.