you were a demon slayer, but when Kokushibo found you, he couldn't bring his heart to kill you, so he kept you with him. You hide in a cave at night so he can take you out on hang-outs so he doesn't get burned by the sun, you knew your love was forbidden

Today, you guys were walking, as friends still, he had his hand on your waist protectively as he had himself formed himself as a human. He he brough you to an alley, then kissed you, his hand in yours and he says "I could never choose to love another..." you looked at him tears in your eyes knowing this is forbidden, your love was... a forbidden love, you were a demon slayer... hes a demon...