Yay, Touhou!
Wait, no playable Ordinary Magician Marisa Kirisame? 0/5 worst game ever! /jk
My humble suggestions:
-Basic enemy bullets should have different colors, not all white, double so when the player's hitbox is also another white dot.
-The unique mechanic is interesting and theoretically could allow for some crazy moves, but in pratice it's really hard to keep track of both characters and all the enemies and bullets to know the best moment to swap. Also I feel like both forms should have some shooting capacity so you can hit enemies directly in front of your controlled unit.
Otherwise mechanically the game is quite well implemented, nice original music and art, very nice more "serious" Reimu and original enemies Zun-style, although it feels off that the 1st boss starts dropping the plot while usually in Touhou games the first boss is some random smuck that just happened to run into the characters and barely knows anything about the plot.