Secret Under Lip

Kwon Chanwoong has just started working in a cosmetic company and was living his job with the awkwardness and passion of a new employee. One day, Chanwoong took the drunk team leader home and accidentally discovered his shocking secret. 
It's Park Youngin, the company's rising star and Chanwoong's role model. So, what is Youngin's secret that flustered Chanwoong so much?