Noooooo, Sakamoto, don't gooooo! We hardly even knew ye. I was shocked that the series came to an end so soon, especially given the tendency of popular manga to go on way too long after the story's told (*cough* magical girl apocalypse *cough*), but I credit the creator with knowing to leave at the top of their game. We still don't know anything about Sakamoto, and that's part of what makes this such a great ending. Where will he go next? To another school in Japan to transform the students there? To the US Mars program for real? Is he some kind of lower-level krishna, sent to return things at that particular school to a place of balance? We will likely never know, and we will be better for it. RIP, Sakamoto.
PS-Loved the graduation speech. Brilliant.