When we were young, my parents remarried and I gained a younger sister, Ichika. She was affectionate toward me and always treated me kindly. Her friendly nature made her popular with my friends, and I found their interactions heartwarming. However, one day I discovered that she had been secretly engaging in sexual activities without my knowledge. I was shocked to see my beloved, special little sister in such a state. Taking advantage of her naivety toward sex, someone had changed her. After that incident, a new emotion began to stir within me.