[Matrixbeast]

Hell, let's expand on the 25 character concept.
Because, ultimately, through including alternate forms, it's actual far, far more than 25 characters.
Since Kuuga Pegasus is more than a green Kuuga Mighty, and Blade King is more than a gold Blade, and Armed Hibiki is more than a red Hibiki, then each alternate form for a character should be treated as their own character.

So Kuuga wouldn't be just one character. By including Titan, Pegasus, Dragon, and Ultimate, he'd be 5 characters, not just 1.

As a result, a character count of 25, with the main characters including the alternate forms you chose, would look more like this:
Kuuga [Mighty, Titan, Pegasus, Dragon, Ultimate]: 5
Agito [Ground, Storm, Flame, Burning, Shining]: 5
Ryuki [Base, Survive]: 2
Faiz [Base, Accel, Blaster]: 3
Blade [Ace, Jack, King]: 3
Hibiki [Base, Kurenai, Armed]: 3
Kabuto [Masked, Rider, Hyper]: 3
Den-O [Sword, Axe, Rod, Gun, Liner, Climax]: 6
Kiva [Kiva, Garulu, Bashaa, Dogga, Emperor]: 5
Decade [Base, Complete]: 2 (Since Kamen Rides would be just copy-pastes of previously made characters)
Double [Cyclone/Joker, Heat/Metal, Luna/Trigger, Half changed forms (C/M, C/T, H/J, H/T, L/J, L/M), Fang/Joker, Xtreme]: 11
OOO [TaToBa, GataKiriBa, LaToraTah, SaGoZoh, TaJaDor, ShaUTa, PuToTyra, BuraKuWani]: 8
Fourze [Base States, Elek States, Fire States, Magnet States, Cosmic States, Super Rocket]: 6
Wizard [Flame, Water, Land, Hurricane, Flame Dragon, Water Dragon, Land Dragon, Hurricane Dragon, All Dragon, Infinity]: 10
Gaim [Orange, Pineapple, Strawberry]: 3

So, that turns 15 characters into a cast of 74.
You can see how that grows to become an issue when you start including all the forms, secondary riders, villains, and etc, now.