There's aspects of all. The Vistamps are tech, made with animal DNA, the inner devil's that most of the Riders need to transform I think were mystic. The show seemed to go back and forth on whether they were actually like Devil's, or just metaphorical inner demons given form. Can't remember if that ever got a definitive answer. And for the Igarashi's specifically, especially Genta, there's a biological aspect considering Genta had Giff's cells put into him, which effected the DNA of hie children as well, making them more powerful than the average human

I think Revice and Geats (unless the Goddess of Creation turns out to be a supercomputer) can be classified as mixed types. So there hasn't been a pure sci-fi show about riders since Zero-One.

New 'Kamen Rider Desast' looks awesome! (Didn't know which thread to put this in)

New Wonder Ride Book 'Gaikotsu Ninja Den' (骸骨忍者伝, lit. 'Skeleton Ninja Chronicle') and Seiken 'Black Storm Sword Jet Black' (黒嵐剣漆黒, no idea how this thing is supposed to be pronounced, although If the Seiken uses on-reading rules, I'm guessing it's 'Kokkuranken Shikkoku' or something along those lines).

https://news.mynavi.jp/article/20230...mages/003l.jpg
https://news.mynavi.jp/article/20230...mages/002l.jpg
https://news.mynavi.jp/article/20230...mages/004l.jpg

He's cool, even though the sword looks half-finished! So Desast is the only character in Saber and one of the few in the franchise to have been two different riders. Incredible!

That got me to thinking, and there's actually quite a large number of characters that became completely different riders.
Shoichi (Agito/G3-X) in Agito, Shinji (Ryuki/Knight) in Ryuki, Takumi (Faiz/Delta) and Kusaka (Kaixa/Faiz) in Faiz, literally half the cast in Kabuto (Every TheBee user became a different rider aside from Mishima), Otoya (Ixa/Dark Kiva) and Taiga (Saga/Dark Kiva) in Kiva, Onodera (AR Kuuga/AR G3-X) and AR Shoichi (Agito/Gills/G3), Eiji (OOO/Birth) in OOO, Chase (Chaser/ProtoDrive) in Drive, Kagami (Gatack/Kabuto) in Zi-O, and several characters in Revice (And Saber) became alternate Riders in Beyond Generations.

Beyond Generation, I did not take into account, I admit. But even in such an extensive list, Desast is unique as a kaijin without a human form. Well, if you add BG, then he shares this title with Vice.
And at the same time, the Imagins have one rider for four! (for five! + villains)

I didn't know whether or not to count Imagins since they're for the most part still variations on Den-O, even if they tend to break the rules and exist outside of Ryotaro's body nowadays :lol

Good list, but there’s one absence: Micchy (Ryugen/Zangetsu Shin)

I just remembered, he was also Delta at one point.

Not sure if this character counts but there's also Yuji Kiba being Faiz for a certain amount of episodes as well as being Kaixa for the show's endgame arc after he killed Kusaka.