K-Drama OST songs, which just stuck in your head: Part 2 - Vocal songs 2

Part 2 of the best K-drama OST songs, I've heard, is here. 

The best vocal OST songs - Part 2:

K-Drama OST songs, which just stuck in your head: Part 1 - Vocal songs 1

This time, I will look into songs, which would not get out of your head even after finishing that certain drama weeks ago. These are the songs, which would always get your attention in a drama, or at least they captivated me.