One of the most entertaining series I have ever watched, all credit goes to Shin Hye Sun, I mean what a performance, such a stark difference from Legend of the Blue Sea and Starngers.
Spoilers
There is a lot of debate over rhe ending but I think many missed several points.
First is that both So Yong and Bong Hwa mind, body and souls intertwined, thus creating the constant conflict between the heart and mind as well as physical reaction.
After the water incident, So Yong was dormant (vegetative) and Bong Hwa was dominant which is why we heard Bong Hwa's voice.
But very early on Bong Hwa was aware how So Yong felt for the King. This is well established which means those feelings were hers not Bong Hwa. But he did bond with the King through her, and this is why the King felt something missing in the end.
After she collapses, So Yong gets her voice back, which means she was the dominant one. That is why her memories all came back. Bong Hwa was seen to be in vegetative state.
So it basically between the concious and sub concious minds of the two souls. Each taking turns to navigate all that is happening around them. In the end So Yong was also sad that Bong Hwa was gone, and she was always aware of him like how he was aware of her. What people dont realise is the bond between these two souls which meant more. In the water, it is So Yong who rushes to save Bong Hwa.
So Yong had always been rebellious but lacked courage, which Bong Hwa gave her. Once she was able to say what is on her mind, she gained confidence and courage.
Bong Hwa on the other hand was seen to be a selfish womansing money minded person. Through So Yong, he learnt to be rightous, which is why in the end his life also changed.
What the writer was trying to comvey is that we humans are always torn between mind, body and soul. There is logic, there is feelings and intuition. We act and react based on that. Perhaps both So Yong and Bong Hwa manifested each other like a hidden personality within our sub concious mind. In that desperation where both of them nearly died, those personalties triggered. In the end they are one of the same people.
This is how I view Kim So Yong - Jang Bonh Hwa.