Definitely not Kou. Kou would be looking for Mitsuba, in any case, and there was no way he knew that his brother remembered the other world too (or that Nene remembered), so separating her from the rest made no sense unless he was a fake luring Nene to not-Hanako
SO MUCH HAPPENING MY BRAIN COULDN'T TAKE IT.. DID KOU JUST DIE???? WHAT WERE THEY SEEING OVER THERE OH GOD