They're just regular idols who can't label what their relationship is... but they both knew its deeper than just "friends". 

Fifteen years after being rescued from monsters that killed her family, Park Chaeyoung joins the Rescue Corps in search for the boy who saved her and will give her her favorite chocolate bar as promised.  However joining this division in the military would reveal not only the truth behind the outbreak that destroyed almost all of humanity, but also cure from an unexpected source that would also eliminate the threat of extinction