This sums it up nicely: http://pastebin.com/YdnpW5HC Here's two of the arguments for those who don't feel like opening the link:
ARGUMENT 2: Ranking System For anyone who has played the demo, you will know that you receive a grade per individual section of an act. This is reminiscent of Bayonetta’s grading system which, if you recall, presents you with those grades at the end of a Verse alongside a total grade. This appears to be similar to that system which presents individual grades along with the total grade at the end of an act (as seen in the large C in the picture).
ARGUMENT 3: Sundowner If you look at the background of the picture, that appears to be Sundowner. Let’s play Devil’s Advocate and propose that there’s a big twist and Sundowner is the final boss. Why on earth is he in the background of an end game screen doing nothing? That would be pretty bizarre as far as end rank screens go.
Now that's what I'm talking about. The guy who played just sucks.
METAL GEAR RISING - THE REBIRTH OF METAL GEAR RAIDEN > BIG BOSS