kuroneko series side story, between Kuroneko 4 and 5. It's cute, and explains a few things from the brothers points of view. Just, it's not the best translation but I can't complain. I read english, and I got the gist of it. If you haven't read the Kuroneko stories, some of this won't make sense time wise and character-wise.
Fukigen Kareshi no Nadamekata