Sekai Ichi no Imouto-sama (Novel)

Maria Lunalady Blackhazel is a beautiful genius with heterochromatic eyes. Oh, that and she's also the leader of a ruthless, multinational private military company that brings forth chaos and destruction. Known as the "Princess of Pure Darkness" around the world, she's the subject of many people's admiration, hatred and fear. But to the main character Siyoung, she's an irrelevant person living in a completely different world. That is, until one day she suddenly shows up and kisses him. "I wanted to see you, Onii-sama..." And just like that, Siyoung was given the world's greatest little sister. *Please note that this is a Korean light novel.