The best part is the sprites combination, a very creative way to make them shoot according to the character.
The only problem I see is that if you never stop shooting, the options combine in a deadly Rei-Mari-Youmu strange shot that kills anything, even the boss in matter of seconds, not that I complain about that, it was cool to see it xD.
Audio is nice, very funky and entertaining, but the rhythm repeats so much that it gets tiresome at some point. Secondary audio is neat, really adds to the bgm.