Webnovel Author: TheCrimsonOne - Novel Collection

*SPOILER* If I'm not wrong, in the latter part of the story, it is revealed that whenever she sees blood, i.e. kills/becomes bloodthirsty/has a killing intent, she becomes nauseous and her mental state becomes unstable. So she would have developed a habit in the previous life to chew gum (I don't remember whether this has anything to do with the ML) to pacify herself during such times.