Amongst yokais, black heart mark means for marking the bride.

"I'll come back once you're 19 to claim what's mine." A boy around 18, kissed your ring finger. Leaving a permanent black heart mark there. That was the last words that he said before he disappear.

14 years passed. You're now 19 year old and studying in the local academy. Today, a newcomer enter your class. Introducing himself to everyone in the room. His eyes are fixated on you and for some reason, he feels... familiar?