Diethard Ried is the head of information, espionage, and public relations for the Black Knights. A graduate of the Imperial Britannia Central University's Faculty of Law, Diethard is a former producer and member of the press staff at Hi-TV, a Britannian television station. Following the appearance of Zero and the announcement of his goals, Diethard joins the Black Knights, wishing to witness and document Zero's campaign to overthrow Britannia. He harbors contempt for the Britannian Imperial Family and its supporters, on occasion commenting to Rakshata Chawla that "Britannia is a finished product" and that he would rather take part in creating something new.
Despite possessing some knowledge of who Lelouch is and of his power, Diethard remains loyal. <hidden by spoiler settings>Only when Lelouch is exposed in front of the leaders of the Black Knights does he betray him. He claims that he would rather have watched Zero topple Britannia, but will settle for a hero's death instead. When finding Lelouch proves difficult following his escape, Diethard suggests reporting him as dead, noting that even if Zero came back he would have no credibility without support. He defects to Schneizel following this. When Schneizel is placed under Lelouch's control, Schneizel fatally shoots him in the heart before he could shoot Lelouch.