I, Zechs Levit y San Foranada, have a younger sister. Her name is Caroline, and she is a very lovely girl with beautiful blond hair and red eyes. It's no exaggeration to say that she's the cutest girl in the world, and her smile can be described as that of an angel. Despite the fact that I can't get enough of her cuteness, my little sister is doomed to die. This world is similar to a game in my previous life called “Tōzai no Isamu Sei-ki,” in which Caroline appears in the game as a rival—A villainess—of the main character, who's a Saint. And as a result, she dies brutally on every route. My beloved sister is going to die? I can't allow such an unreasonable future to happen! That's why I decided to resist fate. I will raise my little sister in a pure and righteous manner, and I will forcefully overcome the numerous death flags. Believing that there's a bright future ahead for me and Caroline. This is the story of me, who became the strongest to protect my sister, Caroline, the cutest little sister in the world.