Use blood as evidence, use body as binding, use soul as evidence. "Why is such a simple contract classified as a forbidden technique?"
"Remember, no matter how much you love someone you meet in the future, or even if we live and die together, don't use this."
"Why?"
"Because, nothing good will happen."
"Then why are you telling me?"
"Because this contract will sometimes let you get the antidote when you are injured..."
"... But I still don't understand, mother."
"You'll understand when you grow up."
"Why... Will you understand everything when you grow up?"
"Because..."