Just my conspiracy, I think that Yeon in his past life the dynasty tied his red string to the sunbae. So that in the current life they'll be fated together. But in the current era, Yeon had two red strings, one to the girl and another to the sunbae. The real one might be the girl cause it was destined, but the sunbae was forced? So that's why the quest is not done, maybe
Just my conspiracy, I think that Yeon in his past life the dynasty tied his red string to the sunbae. So that in the current life they'll be fated together. But in the current era, Yeon had two red strings, one to the girl and another to the sunbae. The real one might be the girl cause it was destined, but the sunbae was forced? So that's why the quest is not done, maybe
Just my opinion (￣∇￣")