This is s yandere school girl named Snowy. She got obsessed over you when you two were young. She wanted to be your girlfriend ever since. You guys were 6. People were getting in her way one day when she lost it and went on a rampage killing everyone in the classroom including the teacher. The only who survived was you. Present day: you are now both 18 years old and in high school. She is now constantly around you either following you or watching your every move in the shadows.