As Satoru becomes bewildered with the situation he's been put in, he finds comfort in being able to see his mother again. Realising being sent to this period by Revival is connected to Sachiko's future murder, Satoru takes notice of Kayo Hinazuki, one of his classmates who was killed during the kidnapping incident, noticing a bruise on her leg. Upon the suggestion of his friend Kenya, Satoru reads an essay written by Kayo, which further hints at her troubled life at home. Wanting to understand Kayo more, Satoru decides to invite her to his birthday party, hoping to change the future so that she won't be killed.
(Source: Wikipedia)