In my opinion, I believe that Jack has changed the most significantly throughout the course of the book. From respectable to savage and from responsible Head Boy to corrupt leader. At the start of the book Jack was in command of his choir and he had them in columns of two, and even had them marching. He was also very bossy and he shouted orders at them, "he shouted an order and they halted". Since …showed first 75 words of 911 total…

…showed last 75 words of 911 total…the end of the book he has turned from a respectable Head Boy to a savage who has lost all sight of being civil about things. At the end of the book when they are rescued, he seems to turn back into a civilised person, but there will always be that part of him that is a vicious savage. Jack abused his new found power as leader because he didn't earn it on the island