Childhood friend?

Yuna is just a normal 20 year old girl. She was born in Seoul. She was treated like a princess for awhile until her parents adopted a little girl from America named Alison. They ignored her just to be with her. The only person she had was her childhood friend Yoongi. They would also talk about things that she doesn't say to her parents. But once Yuna was 10 she moved to America losing contact with Yoongi. But one day they finally meet again but he wasn't the same anymore. What will she feel? How does he feel? What do u think was caused him to be like that?