Onigen is the principal antagonist of the 2009 live-action film Blood: The Last Vampire, the remake of the 2000 animated film of the same name. She does not appear in the original 2000 anime film; she was introduced in the later live-action adaptation as the story's central threat.

Onigen is characterized as the most powerful demon in the narrative and is revealed to be the mother of the protagonist Saya. This dual identity makes her both the ultimate enemy that Saya is meant to destroy and a figure with a deeply personal connection to the heroine. As the main antagonist, she represents the culmination of the demonic danger that runs through the story, and the revelation of her relationship to Saya adds a strong emotional dimension to their confrontation.

Her personality comes through primarily in her speech. She addresses Saya with a mocking, condescending tone, telling her that the idea of becoming more human by killing more demons is naive. This suggests that Onigen regards Saya's struggle for humanity as futile and frames their conflict as ideological as well as physical: Saya hunts demons to affirm her own humanity, while Onigen dismisses that pursuit as an illusion.

Onigen's role in the story is to oppose Saya directly, and the discovery of their kinship redefines the nature of their fight, making it a family struggle rather than a simple battle between hunter and prey. Her abilities match her standing as the most powerful demon, placing her above the other monsters Saya encounters, and her power is treated as the most formidable of the demonic forces in the film. Her development as a character is tied almost entirely to the mother-daughter revelation, which makes her personal connection to Saya the defining feature of her role as the story's final antagonist.