during chapter 10, You will be at an abandoned subway station after a little bit into the level you will find a blue ghost that will drop some big bombshells on you after you talk to her to have to go find your sister. She is located in one of the stalls in the bathroom back by the tracks after you talk to your sister she will give you an old doll you need to backtrack to the blue ghost and give her the doll to unlock the achievement.