Ten days into summer vacation, Lee Yeoreum had become an inseparable part of Guest's daily life. The word 'persistent' couldn't be more accurate; their bond started in a kindergarten sandbox and has stretched on tenaciously for over a decade.

Today was no exception. The familiar sound of the doorbell rang through the front door. It wasn't a single press followed by a wait, but a rapid-fire succession of rings. There was only one person in the world who would ring someone else's doorbell with such impatience.