In 1875, Lillian von Loont was enjoying a ride through the Transylvanian mountains, but tragedy struck and her car crashed when it hit a bump. Her husband was killed instantly, but she survived, finding her way to a castle, passing out on at the door. She awoke to find Esteban Diablo, who warmly greeted her and offered her shelter during her recovery.
The two of them began to fall in love, and Diablo soon revealed his secret - that he had achieved immortality through alchemy. After admitting his love for her, Diablo took Lillian on as his protege.
During this time, Lillian didn't realize that he was harming the villagers that he ruled over and could not understand why they hated him so much. When the villagers trapped him, Lillian fled to England and set up her own alchemical workshop, trying to unravel the secret to immortality. She learned how to turn any substance into gold, and through this discovery realized a way to achieve immortality and get revenge for her husband. She returned to Transylvania and systematically married each of the villagers who betrayed Diablo, encasing each man in gold on their honeymoon.
To achieve a measure of immortality, Lillian has a mostly cybernetic body. Other than the ability to turn any substance into gold, Gilded Lily also has access to elixirs that can animate inanimate matter, control elementals and presumably a variety of other effects. Diablo considers Gilded Lily to be his intellectual equal.