All the Riders except the very last one are obtained via Ragnarok Mode, you have to come across them in the levels ; and since most levels have multiple paths, you won't get everyone the first time. You also have to actually take the recruited Riders in your team at least once in Ragnarok mode in order to permanently unlock them in other modes.
The main villains of Ragnarok mode are unlocked by completing stage 14 (unlocked after finishing Ragnarok mode once and going back to your completed save) a certain number of times.
|Unlockable:||How to unlock:|
|Dark Kabuto||Complete stage 14 five times.|
|Garren||Unlock every other Rider except himself and WCAX, and he should unlock automatically. If not, complete stage 14 again.|
|Nega Den-O||Complete stage 14 three times.|
|Odin||Complete stage 14 six times.|
|Ouja||Complete stage 14 twice.|
|Ryuga||Complete stage 14 four times.|
|Scissors||Complete stage 14 once.|
|Stage 14 and stage select||Complete stage 13|
|W/Double CAX||Once Garren unlocked, go in arcade mode, select the last empty space, press SQUARE(random) and complete arcade mode as WCAX to unlock him permanently.|