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Justin is the youngest child of Nora and William Walker. He always felt as though he's been protected by the outside world by his parents and siblings. When Justin was in his early teens, a family friend named Tucker Booth introduced him to drugs. After Kitty returns from New York to stay with her family after September 11, she confides to Justin her experiences and fears stemming from the event. It was for these reasons that he was compelled to join the Army. When he returned from Afghanistan, he suffered from PTS disorder leading to his drug addiction. Nora blames Kitty for encouraging Justin to join the army, leading to their estrangement.

(wikipedia.org)