Ken Amafuji

According to the mangaka, Kazuna Uchida, the character who grows the most during the story. She even considers that she wrote the manga to follow the growth of this one young man. The novel version of I Shall Never Return is actually written from Ken's point-of-view.
The mangaka most enjoyed writing the character as the bad boy he was when the series started. Eventually he started to change and grew more positive.