Hideo Toyotomi serves as a geological survey site supervisor at Geo Technics under Ayumi Arima. He conspires with researcher Kaori Asakura to steal the proprietary Hyperstone preservation and generation formula, intending to sell it to a competitor for personal gain. To access restricted documents, he manipulates Ayumi Arima.

His background expands in the original adult anime adaptation: He originates from Dela=Grant as the biological son of Keiko Arima (Keitia). After adoption, he becomes the older stepbrother of Sayless, Takuya Arima’s wife. Though receiving Keiko’s full affection, he harbors intense hatred toward Takuya. This drives him to orchestrate a military assault on Takuya’s Dela=Grant home using the Emperor’s soldiers. During the attack, he facilitates Yu-no’s rape and engineers events leading to Sayless’s suicide. His schemes culminate in an attempted assassination of Takuya within the Emperor’s palace, but Takuya counterattacks and kills him.

In alternate narrative branches like Kanna Hatano’s route, he attempts to rape Kanna while under external mind control. Other characters recognize this behavior as uncharacteristic, indicating external influence overriding his typical disposition.

Physically, he is an adult male with dark, green- or petrol-colored hair reaching his ears and grey or brown eyes. His motivations consistently derive from ambition, jealousy, and deep-seated resentment toward those linked to Keiko Arima or the Arima legacy.