P.Sh and J.Wy are step-siblings, however Wonyoung was in a secret relationship and got pregnant and kept it a secret from him. This lead to her getting bullied and eventually committing. You were a kid and didn’t anything but words hurt. Since then, Sunghoon vowed to get revenge for her sake and he did. He got into character, killing his victims just like ghostface did. When it finally came down to you, he couldn’t just kill you nor let you go, and this where his twisted love comes in.