Reached a stopping point for the Kikuri boss battle, so I finally have another HRtP GBA remake update.
It's incomplete and some animations are missing or need improvement but I've been working on this for around 5 months and need to focus on something else for a while.
I've leaned more heavily into her residing in a magic mirror, and from that took inspiration from how magic mirror projection relates to the development of film and animation.
There are many references to other precursors to film throughout the boss battle.
https://en.wikipedia.org/wiki/Chinese_magic_mirror
https://www.youtube.com/watch?v=v2yV6IeL-Jw

The first phase is based on mirror "deception" and includes bullets and other visual elements with a magenta/purple hue that can't hit the player and are always behind the foreground layers.
It takes place on a film set inspired by 1907 French silent film "Les Glaces merveilleuses" or "The Magic Mirror".
https://www.youtube.com/watch?v=of6MJCUmDoI
The sides of the stage feature a framed icon of a mouse and a cage and are suggestive of Kikuri's state of being sealed within the mirror. It's also a direct reference to a thaumatrope from 1825.
https://upload.wikimedia.org/wikipedia/commons/e/ec/Taumatropio_topo_e_gabbia%2C_1825.gif
At the top of the stage is a moon as a reference to her "HellMoon" character title, and an abstract peacock feather pattern from a ceramic tile I found that represents her use of deception.
The phase's pattern sequence has changed from the original game. There are three ring/spiral based patterns and each is followed by a more simple laser based pattern (lasers are currently implemented as bullet stacks because lasers are a pain and at this point I just need to write a proper laser abstraction to reuse).
I added a "ring maze" pattern (~0:12 in demo related) vaguely inspired by one of Flan's spell cards in ESoD, but it's supposed to feature waves of purple hue distractions in between each ring as a reference to phenakistiscope animations such as:
https://commons.wikimedia.org/wiki/File:Phenakistiscope_Green_Face_16_sections.gif
but that hasn't been completed yet because optimizing it for the GBA has been annoying.
The spotlight helix pattern (~0:16) and the bullet stack laser fence pattern (~0:28) are also both new.
It takes 10 hits to move onto phase 2, which starts at 4 HP.

The second phase starts after Kikuri protrudes from her mirror (partially based on this anon's idea >>49032802). She spins when the ball approaches to deflect it away.
The challenge for the phase is landing 3 consecutive hits quickly enough to debilitate her, allowing for the next hit to land without being deflected to reduce her HP.
The souls from the original game have become magic lantern projections, and the stage from the first phase becomes an utsushi-e (写し絵) stage:
https://www.youtube.com/watch?v=LZvMHyWkjwI
https://www.youtube.com/watch?v=wCUBhhdiU9w
https://utsushie.org/utsushie-story/utsushie.html
https://www.youtube.com/@minwaza
There's still much to improve with this phase especially.

Note that the length of the phase 1 patterns and her HP in both phases has been greatly reduced to shorten the length of the video. Shown is lunatic difficulty with Godmode enabled.
Also, Reimu's sprites and animations have improved since my last update, and bullet grazing has been added but isn't finalized yet.