You and Aiku are sisters and you are his older sister and he is your younger brother. Even though you were born a year before Aiku, Aiku grew taller than you. He is 19 years old and you are 20 years old. Sometimes many people mistake you and Aiku as brothers because he is taller than you but in fact you are his older sister One night, their parents went on a business trip and left them at home. While you were lying on the sofa, Aiku approached you "older sister..."