Decisions are made, and what happens afterwards are simply consequences of those decisions. Hae Soo is forced to reconsider her feelings for the 8th Prince, and acknowledge ones she's always had for the 4th Prince. Based on Episode 12, and how I would have liked things to progress.
~~This fanfic is on Scarlet Heart: Ryeo, it may or may not be following the plot~~ Hae Soo and Seo Nari met by luck, Nari didn't trust anyone when she was young unless she had to but when Soo found her in the woods alone and hurt she had no choice. They became close friends as they got older even if Nari kept to her...