Pretty sure that Pink hair is the girl the Yu-na dated in high school as a replacement for Min-ji (shown at the end of chapter 35 and in chapter 36). Woman looks the same but obviously without the pink hair she presumably got after the breakup shown in chapter 36. This would explain why Pink Hair knew about Yu-Na's longtime crush. Would also explain why Pink Hair wants to meddle so much in Yu-Na's love life still. I figure Yu-Na doesn't remember her because of the changes she's gone through but also because Yu-Na probably barely paid attention to her back then. Seemed like she was dating multiple people at once back then to make up for her previously unrequited love.