Don't know if anyone already said this but I think we'll see koishi somewhere in this game between the 4 and extra stage bosses. Maybeshe is accidently causing all the youkai to be angry? Would make since seeing how she can manipulate the self-conclusions. Not to mention she has made it so she can't be hated so prehaps the hate is boiling over in the youkai.
I'm just going to share what a few others all over the Internet have said. If you look at the title screen of the game, it looks like there's a mirror behind Reimu. If you think about the name, Double Dealing Character, maybe this character has something to do with reflections in a mirror. And another shorty~! :D I wonder what she'll look like...
"Haikus are easy But they don't always make sense Refrigerator" -Some shirt
So with sadness in my heart I feel the best thing I could do is end it all and leave forever whats done is done, it feels so bad what once was happy now is sad I'll never love again my world is ending