I understand that I have gone over book(s) limit however his character development is quite thorough throughout the course of these 3 books. In these books and including the feature length movie we see the protagonist Alex Rider living in England with his housekeeper and uncle.
Alex is a normal school boy with nothing much going on in his life however,after the death of his uncle he is thrust into the world of secrets,blackmail and mystery that his uncle was accustom to. Alex in the space of storm-breaker(also a movie),Point blanc and skeleton key has developed into a dangerous, deadly spy for M.I.6.
Throughout the book Alex is an reluctant hero having killed only 3 people through all three books this having drastic effects on him. He also has some supporting characters helping him through his missions one of the Mrs. Jones the assistant to the head of M.I.6 she show a maternal affection towards Alex however, due to her position she is incapable of enacting a more protective assistant to Alex.
In each book Alex has specific targets and challenges to overcome that are not as black and white as the classic genre of good versus evil with each book showing a unique perspective of the villain their motivations and the reasons behind why they do what they do. an example to this would be of the main antagonist in the second book (Point Blanc) Dr. Grief. In the book his home country is South Africa believing in White Supremacy during the apartheid and after years of hatred and anger towards the world for what the had done,So he had concocted a plan which would give him control of the world to build his Utopia.
whereas in the next book and the 3 book in the series Alex had to battle against the foe known as General Alexei Sarov who came as the antagonist similar Dr. Grief however, his reasons behind trying to destroy the World are different and less monstrous with his motivations being the death of his son in the Cold War for a futile effort which angered him drastically. The reasoning behind his assault is not for white supremacy but for communism. Equality for all of the country to grow with fairness between all of the country men and women. This does not justify his actions or power thirst but it does represent that not all things are Black and White.
Finally We come to the main character Alex Rider through out the book we see he craves for a regular childhood with him to be a normal schoolboy to go about his life but the anti-hero aspect rears its head on a number of situations e.g. Alex shows that he has the reluctant hero role that he follows sometimes outright refusing to go on mission for the secret organisation before being blackmailed in a number of missions. He is also prune to violence as stated before he has previously Killed certain people through the first 3 books.
React!