Beatrice Lambert, often called Beth, is an American graduate student who becomes involved in investigating the Hiraga bloodline curse and its alleged connection to immortality. She is physically characterized by her gray hair, which is cut to ear length, red eyes, and the consistent use of hair ribbons. Her appearance also reflects vampiric traits, and she possesses notable psychic abilities.

In the story, Beatrice serves in a domestic capacity as a maid while aiding Gen Hiraga, who has moved into his grandmother's house and experiences recurring dreams tied to his ancestors' tragic ends. Her role is that of a catalyst; her involvement in the investigation triggers Gen's fragmented memories and intensifies his inner turmoil as they work together to expose the clan's secrets. Her background is deeply linked to the supernatural elements of the Hiraga curse, specifically the legend of immortality encoded within the family's genes.

A central enigma surrounding Beatrice involves the discovery of connections between her and a mysterious woman from Gen's dreams, though the precise nature of this relationship remains unclear. She functions as a conduit to the occult dimensions of the Hiraga lineage's secrets and maintains her association with these supernatural elements in the sequel, Zoku Gosenzo San'e. Her development is tied to the ongoing investigation, as her presence directly influences Gen's unraveling understanding of his family history.