How does Piggy change over the course of the story in "Lord of the Flies"?
The beauty and the tragedy of Piggy's character is that he doesn't change. The boys overall become uncivilized. Jack and the choir boys descend all the way into violent savagery. Ralph struggles, not knowing how to behave.
Piggy, though - he is the same uptight, rules obsessed, serious boy when he dies as he was when Ralph first met him in Chapter 1. The beauty is that Piggy presents hope for human kind. For every violent Jack, there is a Piggy, determined to uphold justice and decency. Even scared, Piggy stays true to the dream: 'Ralph -- remember what we came for. The fire. My specs.'
The tragedy is that Piggy can't understand the savagery in others. He underestimates those who have abandoned the rules he lives by. Even after Jack and Ralph have begun fighting, Piggy holds firm to the sense of "right" - not understanding that this has already gone from the island:
"Piggy lifts the shell and says, “Which is better—to be a pack of painted Indians like you are, or to be sensible like Ralph is? . . . Which is better—to have rules and agree, or to hunt and kill? ."
This is when Roger hits him with the rock. Piggy is killed because he doesn't understand violence.
Ralph's final thoughts highlight Piggy's importance as a static character:
"Ralph wept for the end of innocence, the darkness of man's heart, and the fall through the air of the true, wise friend called Piggy."