Lei Lei was born around the 1730s with a twin named Lin-Lin. During this time, ghosts and spirits of the dead arose from their resting places and attacked a rural Chinese village.
After their mother was killed by a powerful demon, she and her sister decided to use Tensei-no-Jutsu to save her, but they used a prohibited sub-division of this technique, Igyo Tenshin-no-Jutsu, and fought to release their mother's soul from the dark. This turned the twins into a jiangshi (僵屍), a type of Chinese vampire. This, however, also gave the two unusual abilities which the two use to fight for their purposes. Lin Lin is actually the ofuda on Lei Lei's hat.
Outside of Japan, the character is known as Hsien-ko.