Solid story with great use of the split personality, but also I don't know what to think about the other personality. I would absolutely be fine if she only is playing around with MC, guess Gu-Eun thinks that way too but if not that could be also turn out to be bad for MC and Gu-Euns relationship, but let's think of the best and she really cares for Gu-Eun and only makes fun of her GF^^.
Also redhead got what she deserved, even if she also loved her, she clearly sayed in her school days that she only used Gu-Eun and the moment it caused "trouble" for her she threw her away in one of the most cruel ways possible. And even after many years she has seen herself as the victim, so yeah that small harry potter wound will be a reminder to be a better person to the people who truly care about you.