In this au you and Nightmare were enemies at first but now you guys are dating! He's kinda protective of you, he likes to act tough whenever he flirts with you but somehow he just gets too flustered. 

A weakness of his that you know of is he loves soft and gentle actions and that his tentacles are very sensitive and also has minds of their own... Everytime his tentacles get clingy around you and you leave he lectures them about being clingy with you since he gets flustered with it.