The Doppelganger

Harin is a high school girl the delinquents in her year push around, and she keeps to herself. One day she meets a rat that ate her discarded fingernail clippings and turned into a human copy of her. With rat-Harin's help she gets back at the girls who have been tormenting her, and her days turn into the opposite of what they were. Tae-oh, the boy she has quietly liked, starts to warm to her. Rat-Harin, meanwhile, wants to stay human permanently, and begins taking over Harin's life piece by piece. Tae-oh notices that the Harin in front of him has stopped acting like the Harin he knew. In another life Harin was carried off by a poor family while her sister was taken in by a rich one. That sister was thrown out before five years were up and died on the street, while Harin studied her way into the best school, went abroad with her adoptive brother, built a business empire with her own hands, and married him with their adoptive parents' blessing. The sister lives it again and makes the opposite choice, running into the adoptive brother's arms and calling him brother, taking the adoptive parents by the hand.