Cthulhu ~Otome no Fureru, Tenshi no Yubisaki~

Reiichi lives a peaceful life with Uko, his childhood friend, and Koinu, his younger sister. But one day, a mysterious girl, August, suddenly appears in front of him and says, "You, my Cthulhu!" His life completely changes... Reiichi knows that he is actually her Cthulhu and gets involved in an epic battle...