@suki To better understand Young-won’s story, you can go back and read Chapter 44.
For more background on Kang, Seo-in’s step-cousin, see Chapters 5 and 19, and for Gyojin, refer to Chapter 35.
It is also revealed that Kang believes Seo-in is responsible for Jae-won’s death, since Jae-won fell from the balcony of Seo-in’s house. This is discussed in Chapter 55.
