From Entropy:
Saerim Lee gets into an accident and loses her memory as a result. When she goes to school, she realizes that everyone is afraid of her, even her two best friends, who tell her that she was a very mean girl who blackmailed people, using their secrets against them. But she realizes that she can order people around because she supposedly knows these secrets, all of which were forgotten in the car crash. She only has one connection to the person she was in the past, and that is her diary. In it, she wrote of the person she was in love with, but never his name. She will stop at nothing to discover his identity!