Ralph appears from the beginning to be fitted out as a leader. He is good-looking, athletic, charismatic, and perhaps most importantly, he has a sense of responsibility and organization. He realises right away the need to create order on the island. At first the other boys generally rally to him. However, although he is level-headed himself, he shows himself progressively unable to control outbreaks of emotion among the boys: fear, and ultimately anger and violence. In the end he is not really equal to the task of keeping order and stability on the island, as more and more boys decamp to Jack's side. He is left alone by the end, and hunted down by Jack's group. In spite of all his ordeals, though, he never gives way to any brutish impulses. He represents basic human decency.
Piggy is a physically unprepossessing character, overweight, short-sighted, asthmatic; the opposite of Ralph in appearance. However, he is the most intelligent and rational of the boys. He helps Ralph create the assembly in which all boys can participate. He has many good ideas about organising life on the island. However, in the end he proves rather naive. He relies too much on reason and intellect, even as he witnesses the boys' gradual descent into anarchy. He is more vulnerable than Ralph in the outbreak of violence on the island, as he is less physically able to defend himself.To the last, though, he insists on civilised values, right up to the moment that his life is ended by an act of savagery. He represents human reason.
Jack, to begin, with, seems quite a reasonable character who accepts Ralph's leadership. However, he begins to rebel; he really doesn't want to obey anyone else. It is not long before he is openly challenging Ralph for the leadership. Gradually he loses all restraint, all semblance of civilised behaviour. He glories in hunting and by the end is hunting down human prey. He advocates a life of wild freedom which brings most of the other boys to his side. He represents anarchy, and in this he is the opposite of both Ralph and Piggy.