Here, I'm going to show some important villains in this story.
Name: Razen.
Villain Name(s): Kamen Rider Dark Decade. Kamen Rider Another Dark Decade.
Other Names: The New Destroyer of Worlds. The Black Decade. Negative Nick. Nick.EXE. Decade.EXE.
Species: Human (may be a different species depending on what dimension he's in).
Home World/Reality: Earth (Dimension-1218).
Equipment:
DarkDecadriver.
DarkDiendriver. (mostly used to summon Kamen Rider clones, Super Sentai/Power Ranger clones, and Monsters via the respective cards as allies, though it's also used as a weapon)
Rider Cards, Sentai/Ranger Cards, and Monster Cards.
Backstory: The physical manifestation of Nick's darkness from all of the pent up and suppressed anger from being bullied during high school. He later separated from Nick and now seeks to kill him, despite Razen knowing that he can't exist without Nick being alive. He later turns into an Another Rider version of his Dark Decade form.
Name: Evolto
Villain Name(s): Kamen Rider Evol.
Species: Blood Tribe (clone).
Home World/Reality: Planet Blood (Unknown Kamen Rider Dimension, possibly the World of Kamen Riders, or the World of Build).
Equipment:
Evol-Driver.
EvolBottles.
Backstory: A clone of the original Evolto, that Razen created using DNA samples of the original that he stole from an organization called; Foundation X, to help him in destroying Nick and his team.
Name: Hundred
Backstory: An evil organization that invades other parallel worlds with the Aurora Curtain. They can copy the powers of Kamen Rider through their advanced technology, allowing the members to transform into them. After the deaths of the Hundred Four, Hundred began to create monsters called; Another Riders. Razen formed an alliance with them to hunt Nick and his team across the Multiverse.
Name: Another Riders
Backstory: Kamen Rider-based monsters that Hundred created to serve them.
Name: Kasshine.
These are the Foot Soldiers.