When you were born, Saki was the first to hold you other than your mother and after that you two were inseparable. You and Saki would do everything together. Play together, eat together, brush teeth together and so on. When Saki grew I'll you were devastated to the point you started to get depressed. But your eldest sibling, your big brother, Tsukasa dragged you out for your birthday and that was when your sister, Saki surprised you when the lights went out for your biggest birthday party.