I don't see the inspiration honestly...
If this is related to colors, I completely understand-- it was mainly a concern of not doing over existing colorways, such as DSA Seafoam or KAM Starry Night (which this set used to be more similar to.)
Otherwise, the novelties and general design prompt were made with both the show and the light novel in mind.
your katakana legends are half-width, is there a reason for this or just a legend entry mistake?
also interested to know why katakana instead of hiragana, was this a personal choice or just stylistic or?
I had issues fitting the full width katakana legends properly without losing contrast, so I went with half-width. Also, I used katakana because, from a design perspective, in my opinion they synergize better with the fonts used. From a functionality perspective, there are many older JIS boards (namely the JIS Dell AT101) which have used katakana legends and have been fairly large serve the JP market. As well as this, it's something different to see on DSA keycaps, which in my mind, is always welcome.