BG9 serves as a supporting antagonist within the Wandenreich organization, designated "K" among Yhwach's elite Sternritter. His origins remain undefined, though theories link him to Arrancar Shawlong Koufang based on visual similarities and shared data collection via physical contact; this remains speculative given contradictions regarding purification mechanics after Shawlong's defeat. Standing 190 cm tall, BG9 possesses a distinctly mechanical and armored form. His body resembles a Western knight's suit, fully encased in thick armor with a mask-like visage featuring four vertical slits and a single glowing eye. Concealed beneath his standard Sternritter cloak lie numerous integrated weapons: tendrils, missile compartments, and a large Gatling gun built into his right arm. He notably lacks biological functions like breathing. His personality exhibits robotic and calculating traits, often speaking in detached, statistical terms and prioritizing data collection during combat. He displays sadistic tendencies, expressing regret over being unable to use stolen abilities against their owners. Despite his mechanical nature, he shows human emotions like confidence and fear, particularly when begging for mercy. During the Wandenreich's initial Soul Society invasion, BG9 confronted Captain Soi Fon of the 2nd Division. He stole her Bankai, "Jakuhō Raikōben," using a Quincy medallion but never activated it. In the second invasion, he targeted Soi Fon again, interrogating her lieutenant, Marechiyo Omaeda, about her location. When Omaeda proved uncooperative, BG9 attacked his younger sister, Mareyo, to provoke him. This led to a confrontation where BG9 impaled Omaeda with tendrils and unleashed his Gatling gun. Soi Fon intervened using her completed "Eternal Shunko," damaging his helmet. BG9 survived, analyzed her technique, and adapted his strategy. He fired missiles from his chest compartments and detonated a large explosion after impaling Soi Fon's wrist. Kisuke Urahara's intervention via a special pill destabilized the stolen Bankai, returning it to Soi Fon, who then struck BG9 directly, destroying much of his face and body. Severely damaged, BG9 used his Quincy: Vollständig ability to resurrect himself and retreated to Silbern. Yhwach deemed him a failure for his defeat. Alongside Sternritter Cang Du, BG9 was sentenced to execution by Jugram Haschwalth. He pleaded for his life, arguing his continued combat capability, but was executed. An oil-like substance leaked from his body during this event. His abilities centered on integrated weaponry: extendable tendrils capable of impaling captain-level fighters, the mounted Gatling gun, and concealed missiles. His Vollständig granted resurrection from near-fatal injuries. The exact meaning of his Schrift "K" and any associated unique power remain unrevealed.