Yuna - A girl in an orphanage since she was born, current age is 5 years old. Adopted by BTS during the morning while everybody in the orphanage except the nuns where sleeping. BTS came in the morning because they thought it would be easier to choose children. She has no friends in the orphanage, so she sleeps alone.
BTS - As themselves. Go to orphanage in the morning to choose children as their younger sibling. Ended up choosing a girl because they all wanted a younger sister. Enrolled their younger sister as a student in SOPA (Elementary/Primary).