I'm confused... Was the movie based on the webtoon, or the webtoon based on the movie?
Because translations are posted here recently but the original webtoon has been out since 2010. you can google for more https://www.mangaupdates.com/series.html?id=60497
https://en.wikipedia.org/wiki/Secretly,_Greatly
Somehow at this point, the main couple also seems like one of the side characters in the story...