Oof, that story. Totally agree about the art and visuals stealing the show; Koishi's art and animations are wonderfully done. Environment is... hm. On the one hand having only the one tile makes the foreground bland, but on the other hand it makes it clearly distinct from the background without being distracting. Really my only complaint on the art front is that the red bullets blend into the background a little too well; maybe if they were brighter?
Game is short, but for a three-day jam that's just a sign of responsible scope management <_<. You managed to cram in everything you needed to say into it despite that length, so kudos!
Boss fight was... hmm. Once it became clear Satori was simply moving about on a track it simply became a game of 'stand where you know she'll stop and spam attack', eating one or two bullets being irrelevant. Though I imagine anything more complex would fall victim to the scope problem. This makes judging difficulty a little... er, difficult though, since the bossfight is the majority of the game.
EDIT: and only now am I realising that 'whispering enemy' was Koishi's third eye orz