Lavinia Whateley is the albino daughter of the wizard Old Whateley. Using her father's incomplete copy of the Necronomicon, she summoned Yog-Sothoth and he impregnated her. Some time later she gave birth to twins, Wilbur Whateley and the Dunwich Horror.
Born in 1878, Lavinia loses her mother to an unexplained and violent death at the age of 12. She is in her mid 30s at the time of Wilbur's birth. She goes missing on Halloween night 1926, at the age of 48. While no conclusions are spelled out in the story, it is implied Wilbur killed her.
Lavinia is described as an unattractive albino woman with the characteristically weak Whately chin. She is uneducated except for the bits of ancient lore she gleans from her father's books and his limited tutoring. Her appearance is described as "slatternly," and she is prone to wild daydreams.
She is likely named after the character in Shakespeare's Titus Andronicus, who is horribly mutilated and raped before being murdered by her father.