Fun gimmick
Could use some rebalancing as currently holding the reimu formation is ridicolously powerfull as it allows you to actually dodge shots while concentrating all your shots into one place.
This also makes it so that the balance barely allows surviving when you’re down to a singular reimu. And recovering from that becomes pretty much impossible as you don’t deal enough damage to get more reimus. Maybe an additional power-system could fix this?
And the Boss is rather… boring. The patterns don’t have much of a bullet hell feeling. And while that charge attack is quite funny it lacks some proper indication, as on my first run when I stacked all my reimus together it just one-shottet everything.
To give the patterns a more Bullet Hell feel I would really really reccommend Sparens Danmaku Tutorials. They are focused on danmakufu but knowing how a proper danmaku engine works does give a lot of the groundwork to make pretty bullet hell patterns.
And on the topic of all reimus being one-shot I would reccommend giving some reimus invincibility in case of another getting hit. Like if a reimu gets hit 10% of the reimaining get invincibility, and on the next one being hit another 10% get invincibility. And so on. Of course the actual amoutn would need to be tweaked but it would make things a lot more forgiving.