Alex is probably the character that changed the most in the book. He's the main focus so it's hard to see many changes in other characters. Alex matures throughout his adventure because he is challenged every day to keep his cover. Without his level of maturity, he wouldn't have made it through the first day at the Stormbreaker plant. Of course he has to change to act like the boy he is impersonating but he gains knowledge all through the book and this causes some major changes in his thinking.