I'm not that familiar with KnM (although I will be soon), but the Kaon-Himiko romance is strong for me. I don't think it needs a long build-up. It was well drawn in just a few scenes. I fear that extending this show to 26 episodes would be too much for the story to bear. I think the speed keeps us from worrying about everything making total sense. And keeps this a series without any filler.