Theo Himezuka, nicknamed Seo, navigates dual identities as a half-British, half-Japanese operative in Shinjuku’s Nocturnal Community Relations Division. His dyed-black hair masks natural blond tones, a deliberate choice to blend in, yet his androgynous bob cut and delicate features persistently spark gender ambiguity. Defying his youthful appearance, Theo’s 38 years stem from witch-blood lineage traced to the Himetsuka Clan—aristocratic architects of the division. At 17, he relocated to Japan, later forging a pivotal partnership with Reiji Senda during a kitsune spirit encounter at a temple. This collaboration propelled both into the division alongside Kyōichi Sakaki and Arata Miyako.

Though born into the occult-practicing Himetsuka, Theo lacks their innate spiritual gifts, instead channeling brilliance into engineering anti-Another weaponry. Dubbed the "Genius Inventor," his creations—barrier tape to immobilize supernatural entities and Ibn Ghazi spray to expose them—form the division’s tactical backbone. A rift with his clan emerged after uncovering their torture of Anothers beneath the ancestral estate, driving him to solitary apartment life.

Professionally composed, Theo privately bristles at colleagues like Kyōichi weaponizing his androgyny to dodge romantic pursuits. His analytical rigor fuels a relentless drive to catalog supernatural phenomena and refine tools for interspecies diplomacy. The "Fukurokouji to Anoko to Ore to" OVA hints at his formative bond with Reiji, including their retrieval of an enigmatic box tied to occult forces—its purpose still obscure to Theo. Evolving from technician to linchpin mediator, he bridges human and Another relations through pragmatic innovation, leveraging clan legacy without its mysticism.