Board Thread:General Discussion/@comment-19267090-20141125000403/@comment-11188061-20141125090131

Obviously SSS rank will be the humanoid Grimm (well actually, they'll start popping up around A rank and above). After all, the only thing a Grimm does as they grow older is to hone their killing skills until humanity is completely exterminated.

So who is to say that the Grimm didn't pick up killing skills from humans, who are so efficient at slaughtering them. Who is to say they didn't become human (or human turning themselves into Grimm through some crazy experiments or... whatever).