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DOUBLE
- Two guys who transforms into one rider with a cool-ass bike
- Uses Gaia Memories stolen from the Antagonists.
- Fights against antagonistic group called the Museum who uses Gaia Memories to wreak havoc on the city.
- Has a berserker state called FangJoker.
- Aided by a rogue member of the Museum.
- Has a shitload of forms on account of the Gaia Memories with Half-Changes.
- Has a partner who starts out initially wary of the main rider. (Ryu Terui/Accel).
- Has a main female lead who starts out annoying then becomes a good friend/companion. (Akiko Narumi)
OOO
- One guy who transforms into one rider with a cool-ass bike.
- Uses Core Medals stolen from the Antagonists.
- Fights against antagonistic group called the Greeed who uses Core/Cell Medals to wreak havoc on the city.
- Has a berserker state called PuToTerano Combo.
- Aided by a rogue member of the Greeeds.
- Has a shitload of forms on account of the Core Medals with Combos and random combos.
- Has a partner who starts out initially wary of the main rider. (Shintaro Gotou/Birth2)
- Has a main female lead who starts out annoying then becomes a good friend/companion. (Hina Izumi).
The similarities DOES NOT END THERE. The biggest similarity is this.
Has a shitload of merchandise to sell to kids.
There. my review of Kamen Rider Double and OOO.