WMG / Kamen Rider Blade

The creatures on the Undead cards look nothing like the actual Undead...
...because several cycles ago, the battling Undead grew so powerful that they caused a global cataclysm, and the winner back then, Human Undead, demanded that all Undeads' powers be nerfed down till they were all humanoid-sized. It's the very reason none of the Undead animals are dinosaurs.
  • Jossed, we had a flashback of the original battle, and they were the same.

The reason that all of the undead are humanoid, is because the human undead won the previous battle.
The undead take on a form roughly similar to the current dominant species on earth. If, for example, the stag beetle undead had won, we would be watching a bunch of beetle-monsters duking it out instead. Also, as with the above theory, the images depicted on the undead cards are likely their true forms.
  • Jossed, they were humanoid during the battle fight original.

The Clow Cards were based on the entire undead cards except for Joker
All because the creator does not like fugly animals, he revised his version with mostly Bishounen Line elements and beat plagerism in the process!

The entire show is actually a Show Within a Show.
The novel "Tasogare", which details what happened after Blade, reveals that the novel Shirai was writing throughout the show about the Kamen Riders became very popular, even becoming, you guessed it, a live action TV drama with tons of merchandise to its name. What we're watching is that very show.

Glaive is the seres's equivalent to Big One
As acknowledged during the Hero Change battle in Kamen Rider Super Sentai: Super Hero Taisen between Decade and Gokai Red, Blade's poker Motif makes it the Rider counterpart to JAKQ. All the other riders have counterparts (Blade has Spade Ace, Garren has Dia Jack, Leangle has Clover King, and Chalice has Heart Queen), so that would leave one of the Missing Ace riders as the series equivalent to Big One. Glaive has the most appearances outside of the movie, so it would make the most sense for him to be the series equivalent to Big One.