The four main characters in Lord of the Flies, by William Golding, each represent some aspect of mankind.
Ralph is the first character we meet, and from the beginning it is clear that he is symbolic of man's physical being. He swims, he jumps, he leaps, and, when he realizes there are no adults on the island, he stands on his head--more than once. The boys elect him leader simply because he looks like a leader. Jack wants to kill Ralph in the end because he is the last (physical) barrier to Jack's complete savagery.
Piggy is the next character we meet, and he is certainly not physically appealing. Piggy is fat, wears thick glasses, and has asthma attacks; he never tans and his hair sticks up in little tufts. The first thing Piggy does is ask questions. He wants to know what happened to the plane after the crash, and he is the one who teaches Ralph the power of the conch (all Ralph wants to do is make funny noises with it, of course). When the boys start to gather, Piggy tries to get everyone's names and wants to be sure someone keeps track of the boys. He is the one who first understands the threat Jack poses, particularly to him and Ralph, and he is the one who wishes there were adults on the island. Piggy symbolizes intellect, and when he dies, all that is left is the body fighting against unchecked human nature.
The first time we meet Simon, we learn that he is a "fainter." He is the one the littluns ask to help them get fruit when they cannot reach it, and he is the quiet voice of inner knowledge. He tells Ralph he thinks Ralph will live to get off the island, and he is the prophetic one who understands that the only beast any of them have to worry about is themselves. Simon represents the spirit or soul of man, and he is the first of the three characters mentioned so far to die. Golding's point, of course, is that the soul is the first thing to die when human nature is left unchecked. The final image we have of the sensitive Simon even indicates a Christ-like figure with nature's halo around his head:
The water rose farther and dressed Simon’s coarse hair with brightness. The line of his cheek silvered and the turn of his shoulder became sculptured marble. The strange attendant creatures, with their ﬁery eyes and trailing vapors, busied themselves round his head.
The final character is Jack, and he represents the part of us all that we refer to as human nature--the instinctive compulsions (often negative and self-serving) we have which are normally kept in check by the boundaries of law and civilization. He is the first to be referred to as a savage. Jack paints his face for pig-hunting, creating a kind of savage mask:
[T]he mask was a thing on its own, behind which Jack hid, liberated from shame and self-consciousness.
Jack is the cruel, selfish leader who wants things his way. When the only impediment to his complete control of the island is Ralph, Jack orders his savages to kill Ralph. If he had succeeded, Jack would have been able to do anything he wished without any restraint or restriction.
Golding traces the descent of man without any outward control or authority by using these four characters in this novel.