Everybody knows the young, kind-hearted vice president Koo Chanwoo of the iKON Mall which belongs to his family. Its CEO was no other than his not so friendly brother Koo Junhoe. Let's just say the ways they treat other people are different, but they are close. Usually, business men aren't widely known by the media. But since they have the look, the money most of all they're only 24-25 years old so they are exceptions. 

Yunhyeong, a hyung Chanwoo knows, whose parents owned a cafe, but 2 years ago they decided to go travel around the world, have left the cafe for their 3 sons:

- Jinhwan, the oldest son who is a writer, only helps the cafe when he's not dealing with the books he has to write.

- Yunhyeong works as a normal salesman in a fashion shop in iKON mall, he designed all the clothes there under the name YS which known by the world as the most talented designer. 

- Hanbin the youngest, who is now the new owner of the cafe because his brothers didn't have much time for it also because he's an awesome barista, has renamed the cafe M&M.

So nobody knows who YS is since he's never showed up. Actually no one acknowledges the normal Yunhyeong even when he spends almost his time of the day at the mall, except his boss from the YS fashion shop and the dongsaeng he knows.