Neko-ane.

When Midou Ginnosuke lost both parents at the age of ten, his grandmother decides to take him in. He goes to live with her and her pet cat, Anzu. Life was peaceful for a time and all was right with the world, until he wakes up one morning and discovers that the girl with cat ears lying beside him is none other than Anzu, who has transformed into a nekomata (two-tailed monster cat).