Source: FANDOM Wikipedia

Within the Multiverse, Another Decade has the ability to access several Another Worlds, which are alternate versions of reality in which past events are altered based on the regrets of their prime universe selves, like how Sougo Tokiwa's classmate Nishimura lost a race in the real world, but won it within the personal reality of the Another World.

Using this effect, Swartz creates worlds where the battles of defeated evil Riders had different outcomes, effectively bringing them back to life, like how Katsumi Daido defeated Kamen Rider Double during their climatic battle in his version of the reality of that world. However, all those Riders, regardless of their true nature, faithfully serve Swartz in all his schemes.

Known Residents:

Kamen Rider G4

Kamen Rider Rey

Kamen Rider Dark Ghost

Kamen Rider Fuma

Paradox Roidmude (Kamen Rider Dark Drive & Another Drive)

Katsumi Daido (Kamen Rider Eternal)

White Woz (Kamen Rider Woz)

Kamen Rider Yuuki