First of all i'm so sorry for leaving you all confused about what yeri is and second thank you so much for your comments <3

Yeri was an angel and her duty was finding happiness for mark. Kind of like a guardian angel. But she made a mistake and fell for mark, that was also why she said falling for mark was her flaw cause as an angel she wasn't suppose to fall for a human. When mark and donghyuck finally got together, when mark found his happiness it was time for yeri to go back to heaven but she wanted to say her goodbye to mark so she shapeshifted to a butterly and talked to mark then flied to heaven ^-^