For people confused, I think I offer a summary to help:
Chapters 0 & 1 introduce us to both girls and show us that they're both crazy or psycho in a way. YanJin is the type of person who appears to not care if you abuse her or threaten to kill her since it's basically what wants. She finds the threat to her life exciting which is why there were all the "Kill me" & "Please Kill me" words popping up around her when the knife was held to her neck. I didn't catch the other girl's name but she just likes to fight and beat people up to the brink of death to establish her power/dominance over them. It states this is a Psychopath x Psychopath relationship so both girls are possibly going to use forms of violence against others to progress & protect their relationship.