Secretly, Lovely

Ri Hae Jin, the youngest secret agent of North Korean history, was send to a small town in South Korea and disguised as a High School student. He was trained by elite special forces since his youth with the goal to become a part of a group of spies called the 5446 Corps. After meeting his lifesaver and team leader Won Ryu Hwan, also known as Bang Dong Gu and Lee Hae Rang, the son of one of the most important officeholder, they got an order: to commit suicide.
But the three resisted and soon they were chased from their own comrades. When they fled, they got critically injured. Fortunately Seo Soo Hyuk saved them. During their stay at the hospital, Soo Hyuk's daughter, Minyoung, took care of Hae Jin. He never thought that he would fall in love, but she changed everything.
[ATTENTION! DON'T READ IF YOU HAVEN'T WATCHED THE MOVIE YET. THIS MIGHT SPOILER AND MAKE YOU REGRET FOR LIFE. I have warned you, thanks.]