Don't forget my boy Kivat Bat the 3rd.

Huh, so I guess we were wrong to guess his form based on the image on the sword. Granted I have a feeling that might come into play later.

Still this is pretty unique, and hey! A 2nd Quarter/New Year "Rider" that isn't an enemy but helps out? That's actually a first I think since Mach?

They were sentient drivers or items but not really "Riders" themselves. Riders were Gold Drive, Ark one etc...

Here when you say "KR Saikou" the image you should picture in your mind is not humanoid but just the sword. I think that's a first (Well, there was Lazer who was a bike I guess. Not humanoid).
But I have no doubt that any figurine branded "KR Saikou" will be released as a humanoid "Saikou Shadow" holding the sword. But technically, the rider is just the sword x)

Honestly, I really love some of the photos included in these scans, they look like older shows that were shot on film. I miss that aesthetic.

Alright it took me a while with a lot of "zoom an enhance"-ing, but I think I figured out what wonder ride book Saikou uses.

The book itself is named "Kin No Buki, Gin No Buki" which translates to "Golden Wepaon, Silver Weapon" which I think is supposed to be referring to Aesop's "The Honest Woodcutter":

That would make sense.

thought the final form would be light themed

I really like Shadow Saikou's design, it's very simple but it still looks awesome and I like how it resembles a de-sparkled version of the figure in the book. It's also neat that even though we had a villain associated with Darkness, now there's a warrior of Shadow who seems to be on the side of good (unless Yuri turns out to be part of the evil within SOL).

does this mean a human will eventually possess this sword to be a full time rider?

First post here.
This is incredibly interesting. Though the shape of the "Kin No Buki, Gin No Buki" Wonder Ride Book is rather odd, from what I see of it. It doesn't look like it would make a part of the Spreads that the other Wonder Ride Books do. What a weird shape.