He planted a love charm in her, so that she could only love him and never leave him in this life.
After becoming a demon, he lost his memory, abused her to his heart's content, played with her, and married someone else.
Someone came to save her, but she couldn't escape. She could only love him, but he didn't love her anymore.
"You can only love me."
He gave his life in exchange for her freedom.
"I will always be my sister's little follower."
But all this is the conspiracy of the people behind the scenes.
Intention: redemptive love