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.
This is a mash-up of the words 'Kismet' meaning 'fated' or 'destined' and 'Nemesis', which i hope we all know means 'rival' etc
so together, a Kismesis, (or plural, as in 'a pair of Kismeses' taking after the plural of nemesis) is your fated rival; or even, some would say, your Serendipitous Rival. Kismessitude is also sometimes called 'Black Romance' or 'Caliginous Romance' (calidinous meaning misty, dim; obscure, dark) in that it is dark and hateful, so lets not forget the romantic aspect, as in competing with someone so often, tensions run high... including sexual tension (eyebrow-wiggle)
now that you know what a Kismesis is, go get your black-mack on
A lot of people think that Naruto and Sasuke are in fact Kismesis, as not only do they do their best to out do each other at every turn, but they also kissed at least once.