A Kismesis is the term used to describe deep, deep hatred for someone. This is the person that you could, and will likely, hate for all your life, no matter what they do. This, however, is a romantic hate, usually involving many hate-snogs and other violent, but romantic, acts such as that.
A perfect example is Spades Slick and SnOwman. They hate each other to a degree at which not achievable by most beings, and is the most powerful pairing of kismesis seen yet.