Diethard Reid is a supporting character known for his sharp intellect and morally ambiguous nature. A tall man with blonde hair worn in a long ponytail and indigo eyes, Diethard is often seen in a distinctive carmine turtleneck. He is a graduate of the Imperial Britannia Central University's Faculty of Law and originally worked as a producer for the Britannian television station Hi-TV. Disillusioned with the controlled, stagnant nature of Britannian media and aristocracy, Diethard possesses a cynical belief that there is no such thing as objective information; all news and biographies are inherently shaped by the creator's intentions. This perspective drives his sole ambition: to personally witness and document the creation of a new, tumultuous chapter in history.

Diethard's primary motivation is not justice, freedom, or loyalty to any nation or ideology. Instead, he is a zealot of grand narratives, seeking a leader who can bring about sweeping, chaotic change to cure what he sees as the world's stagnancy. He is first drawn to the masked revolutionary Zero after witnessing his dramatic rescue of Suzaku Kururugi. Fascinated by Zero's potential to overthrow the existing world order, Diethard offers crucial military intelligence to the fledgling Black Knights, thereby securing his place in the organization. With Zero's tacit acknowledgment that Diethard's predictable desire to "make Zero a god" makes him easy to control, Diethard is entrusted with a pivotal role, becoming the head of information, espionage, and public relations. In this capacity, he is instrumental in transforming the Black Knights into a structured organization, expanding their intelligence network, and managing their public image.

Diethard's personality is marked by a chilling pragmatism and a lack of conventional morals. He views people as pawns in a grand game and advocates for the most ruthless courses of action. He encourages the assassination of Suzaku to prevent him from becoming a symbol for the Britannian occupation and manipulates Kallen into attempting the act behind Zero's back. He takes sadistic pleasure in broadcasting the footage of Princess Euphemia's massacre to incite riots across Japan, caring only for the spectacle and its impact on his documented narrative. Despite his sharp perceptiveness, which allows him to deduce Zero's involvement in secret operations, his loyalty is conditional. He serves Zero not out of devotion to his cause but because he finds Lelouch's rise to be the most captivating story available.

His pivotal development occurs when Lelouch's identity and Geass power are exposed to the Black Knights. Though he initially defends Lelouch, arguing against the betrayal, Diethard ultimately abandons him when he decides that Prince Schneizel el Britannia offers a more interesting path to global upheaval. He defects to Schneizel's faction, eager to document a plan that involves mass destruction on an even greater scale. In the final conflict, when Lelouch turns the tables and places Schneizel under his control, Diethard's obsession reaches its peak. Enraged that his chosen narrative has been usurped, he attempts to kill Lelouch but is fatally shot by the Geass-enslaved Schneizel. In his dying moments, he does not seek redemption but rather begs Lelouch to use his Geass on him, viewing it as the ultimate power he wishes to experience. Lelouch refuses, deeming him unworthy of such an end.

Throughout the story, Diethard's key relationships are defined by his professional obsession rather than personal connection. He sees Zero as an instrument of historical change, Schneizel as a new, more interesting protagonist, and everyone else as tools or an audience to be manipulated. While he possesses no supernatural abilities like Geass, his skills are formidable. His notable abilities include a sharp, analytical mind, mastery of media manipulation and propaganda, and exceptional competence in espionage and organizational management, making him a powerful, if ultimately self-serving, asset to any cause he joins.