His character development is probably the best in the books. In the early books of Trials of Apollo he was selfish and narcissistic, but later he became more human. His emotions became changed, he developed a conscience, he began to regret the bad things he did as a god. He changed throughout the series, and his character development peaked in The Tower of Nero. I always thought of him as a decent character, but I now know he's one of my favourites