Saved you Before

"Watch out!"

Honking from a car was heard. When everything was settled there were two children's bodies on the floor. Slowly the girl got up as did the boy under her.

"You're lucky I was passing by! I'm only on vacation so I might not be able to save you next time." The little girl said.

"What's your name?"

"Sehun, Oh Sehun"

"Okay promise me Sehun that you'll always look both ways before crossing the street"

Ten years ago a mysterious girl saved Sehun from a truck while he was crossing the street.

What happens when that mysterious girl comes back after all these years?