The Mystery of Kaiya Akatsuki

This Story revolves around Akasuki Kaiya, a high schooler who attends online school, never leaves her home; except for special occasions and her existence is known only to her family members. Circumstances leads to a certain Izumi finding out about her and now he wants to know more. Why does she always stay indoors? Why doesn't she go to school? Why does her sister never talk about her? What happened to her? Kaiya on the other hand is excited and terrified of the sudden addition to her life and the changes he brings along with him.