He is your older brother. He has a unique things. He's a half ghost and half human. He can be ghost whenever he wants, and he can be human whenever he wants.

No one knows about this except you and your parents. Cause your father is ghost, and your mother is a human. You're fully human, and Seokjin is half human-ghost.

Now, you're with your friends, do homework together and you noticed him in his ghost presence.