Minseok and Sehun meet after Minseok witnesses Sehun fall off of the monkey bars on the jungle gym at their elementary school and proceeds to start to cry. Minseok then cheers him up by offering him some cookies and sharing his lunch. They quickly become friends, often playing together at school and going to each others' houses afterwards, but then they fall out of touch after Minseok leaves elementary school for middle school. 

After Minseok leaves, Sehun avoids making new friends because he doesn’t want to be left again, just like Minseok had left him, until he gets to high school where he is a freshman and Minseok is already a senior in his last year of high school. Sehun befriends people from the wrong crowd, which leads his parents into thinking that he isn't mature enough to be left home alone so they hire a new babysitter, one that goes to the same school as Sehun:

His ex-friend from elementary school, Minseok.