"My Sister is a Demon" is a slow-shrink, slow-growth story that will end up being around 20-25 chapters, and it tells the story of Clayton Williamson, a 17-year-old high school senior who was raised by his mother after his father abandoned the family when Clayton was 5. Turns out, he abandoned the family to be with Kazehana Asmodeus, an alluring, tall, voluptuous woman who is actually related to Asmodeus, the ancient Demon of Lust. In exchange for 13 years of servitude, Kazehana promised Clayton's father fame and riches. The thing is, though, that as soon as Mr. Williamson entered onto the contract, any time he felt lust for Kazehana, he would shrink a little. And he did feel lust for her....a LOT...and he shrank accordingly.

Our story picks up right after news comes that Mr. Williamson has died. No reason is given for his death. Clayton and his mother have to go to Kazehana's house to answer for the contract -- Mr. Williamson has died after 12 years, and 1 year is left of his contract. And either Clayton or his mother must take his place.