Sekido was a demon a Hantengu clone demon, he had a wife in his human years and her father approved of him. They died, he lots everything that meant family to him.

He killed everyone in the room who told him the news then left out of anger and then the one and only Muzan turned him into a demon.

You were reincarnation of her but you didn’t know who Sekido was, he was about to kill you for food, but then he stopped. Sekido: “… You look way familiar..”