Ren Fujii, the main protagonist, sports shoulder-length blue hair and matching eyes, mirroring both Mercurius and Lotus Reichhart. His eyes manifest Mercurius's caduceus symbol when activating Die Ewigkeit. Personality-wise, he is reclusive, crude in speech, and sarcastic, frequently teasing his childhood friend Kasumi Ayase. A slightly perverted side exists but remains rarely visible. His defining trait is a powerful desire for routine and safety, crystallized in his Briah ability's mantra: "I want to live this moment forever." This contrasts sharply with his deep aversion to blades, born from witnessing Shirou Yusa kill Kasumi's father, though he eventually overcomes this after meeting Marie. He displays tsundere traits, acting coldly yet caring intensely for close associates like Marie. Created by Mercurius (Karl Ernst Krafft) as a substitute to fill Seat XIII of the Obsidian Round Table, Ren resulted from merging parts of Mercurius's soul with Lotus Reichhart's consciousness. Born when the Round Table's seats were filled, he was entrusted to Kouzou Ayase, a descendant of ODESSA members, who claimed Ren's parents perished in an accident. Ren grew up with the Ayase family, including Kasumi, and befriended Shirou Yusa. At age six, Shirou murdered Kouzou, and Ren assisted in covering it up. Two years before the main story, Ren, Kasumi, and Shirou relocated to Suwahara City. Two months prior to the narrative, Shirou provoked a brutal rooftop fight by threatening to expose the murder, hospitalizing both. During the common route, Ren leaves the hospital aiming to resume normal life but suffers nightmares linked to a guillotine at a museum exhibition. These coincide with real-world decapitations orchestrated by Kasumi under Mercurius's influence. Following encounters with LDO members Wilhelm Ehrenburg and Rusalka Schwagerin, Kei Sakurai saves him and trains him to wield his Holy Relic—the guillotine housing Marie's soul. This Ahnenerbe enforces instant death via decapitation, bypassing immortality or regeneration. Ren masters its stages: Assiah (close-range cutting force), Yetzirah (manifesting a red-black scythe replicating Marie's curse), and Beri'ah. His primary Beri'ah, Eine Faust Overture, accelerates his personal time for extreme speed. Alternative versions include Eine Faust Scherzo (effective against undead) and Eine Faust Finale (a Hadou-type granting enhanced strength and time distortion), the latter requiring Rea Himuro's intervention to control his rage. His development involves confronting his artificial origins and role in Mercurius's plans. Initially clinging to normalcy, he embraces supernatural responsibilities to protect allies. Key moments encompass freeing Kasumi from the guillotine's influence, wielding relics like Thrud Walküre when separated from Marie, and grappling with revelations about his creation. In Marie's route, he achieves Atziluth—Res Novae: Also Sprach Zarathustra—freezing all existence in eternal stasis to safeguard his loved ones. Post-series, he ceases aging but remains separate from reincarnating allies. The Rea route sees him purge Mercurius's influence entirely, becoming a true reincarnation of Lotus Reichhart before fading peacefully. Guilt—particularly over Kouzou's death—loyalty, and the conviction that "the dead should stay dead" shape his actions throughout his journey. His combat style evolves from a reluctant brawler to a deity capable of challenging omniversal entities, yet his core motivation endures: protecting his found family.