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The Hero of Truth and the Hero of Ideals
The Hero of Truth (Japanese: 真実の勇者 Hero of Truth) and the Hero of Ideals (Japanese: 理想の勇者 Hero of Ideals) are characters from the backstory of Pokémon Black and White, and who appeared in both White—Victini and Zekrom and Black—Victini and Reshiram.
In the anime
They are twin brothers and are the sons of the King of the People of the Vale that lived one thousand years ago in Kingdom of the Vale.
One day, the two brothers began to fight over their beliefs and started a war in the process. This devastated the kingdom and caused the Dragon Force energy that was enriching their kingdom to become destructive and chaotic, causing it to destroy the land.
The chaos the Dragon Force caused forced the King to use Victini's power to move the Sword of the Vale to the location that would eventually become Eindoak Town. Despite managing to calm the Dragon Force, the King unfortunately died soon after, accidentally leaving Victini trapped inside of the Sword of the Vale's barrier. Reshiram and Zekrom, exhausted from their battle, reverted into stones known as "Dragon Stones". Guilt ridden, the brothers took and hid the stones in a secret cave hidden deep underneath the castle.
In the games
The Heroes of Truth and Ideals are mentioned in Black and White, and are usually called "The Twin Heroes." According to legend, they once shared a powerful dragon, but could not decide whether to pursue truth or ideals. The dragon split into two parts, Reshiram following the Hero of Truth and Zekrom following the Hero of Ideals.
At the beginning of the games, ZekromB or ReshiramW resides in Dragonspiral Tower as the Dark StoneB or Light StoneW and is called upon by N to create a world where people and Pokémon are separate. The player is tasked with resolving the history the twins began in the past by finding ReshiramB or ZekromW in the Light StoneB or Dark StoneW and reviving it.
It is also said that one of the Twin Heroes built the Relic Castle, which became the center of civilization in Unova. Presumably it is the Hero of TruthB/Hero of IdealsW due to the Light StoneB/Dark StoneW being found here.