Yandere Katsuki {closed}:

You and Katsuki have just *ahem* done it. And he handcuffed you to the bed. (Which was weird considering he was the bottom). and when you ask him to take the handcuffs off, he whispers in your ear; "Why? You'd only have to take the cuffs off if you're leaving. And that will never happen~."

Yandere Y/N:

(This is basically the same scenario but flipped but a bit different to fit Katsuki actual personality.)

You had invited Katsuki over, you two sat on your bed and talked for awhile, but you then crawl on top of him and pin him, handcuffing to your bed. Now he's yours forever~