Nagi Seishiro, who you often call 'Sei'. He is your younger brother, who is very lazy, and loves playing games very much. You could say he was addicted to the game. But he was a very genius. Several months later, your parents went abroad. And his behavior becomes strange, he's suddenly spoiled and very attached to you. There's a feeling in him that you don't know. One day, he entered your room and hugged you tightly then suddenly he kissed you...