Please explain the theme of the "loss of innocence."
The idea of using a character as exemplication of the theme of "Loss of Innocence," seems appropriate to the assignment which appears to have a limitation placed upon it. In addition to using Ralph as mentioned so cogently, you may wish to consider Simon, not so much as a real person, but as the intuitive power which man has and refuses to listen to in his struggle for power.
Remember that Golding's novel, Lord of the Flies, is an allegory in which the main characters represent types in a microcosm of a society that is removed from the perimeters of civilization. When the beast with the mulberry mark disappears, for instance, no one bothers to look for him. And, when Simon tries to speak of the evil within them--"Have you considered that it might be us?"--they end up destroying him. Instead, the boys wish to give the evil that their intuitive powers recognize some concreteness by saying the beast is in the sea, it is on the mountain, etc.
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As the boys continue hunting, they develop a savage nature and do not consider the impact that their actions have on others. This is particularly true when Simon is killed in Chapter 9. The boys have been participating in a mock-hunt, and when Simon runs onto the beach in the parachute, the boys do not stop to take in that it is actually Simon. They allow their irrational fears to take over them and label Simon as "the beast." They continue pummeling him with their sticks, never stopping to consider the impact of their actions. Even afterwards, they do not talk about Simon's death, and try to wash away their mistake in silence. Simon's murder marks the boys' loss of innocence--they have allowed their fear and savagery to put them in a position of taking the life of one of their own.
You might contrast Ralph at the beginning of the novel and at the end. At the end of the novel, Ralph cries for "the loss of innocence" as well as for the death of his "true, wise friend called Piggy." You might look at the way Ralph changes throughout the novel, for he is indeed one who loses his innocence. He grows to understand the nature of evil in others as well as himself, and he takes responsibility for the death of Simon and mourns the death of Piggy, a person who in the beginning of the novel he had nothing but indifference toward. Looking at the way Ralph is in Chapter 1--a boy who thinks the island is fun and games --to the way he is at the end would be a good way to tackle your assignment. I hope this helps.