Without Yunjin-kim the character of Sun would not be the same. An actress was needed to portray a woman with severly high levels of empathy and strengh as well as the ability to come across as a loyal wife yet independent woman at the same time, which is never an easy task to fulfill.
She makes the role look effortless, a sign of any great actor.
Kim and her co-star who plays Jin are so good in their respective roles that you could easily believe they are a real life couple.
As the series progresses I expect her charcter to show her potential and come into the main focus much more.
Besides being a great actress she also looks wonderful too.
The sun always shines.