Reshiram is one of the legendary dragons mentioned in the myth of Unova’s founding, and represented Unova’s ‘Truth’. Many years ago, it has turned itself into the Light Stone for some reason, which was later uncovered by Clay at Desert Resort together with the Dark Stone. The Gymleaders kept the Light Stone’s discovery and secret and hid at the Nacrene Museum. After Team Plasma snatched away the Dark Stone, Brycen recovered the Light Stone, which chose Black as its trainer. During Black’s subsequent encounter with N, Reshiram temporarily materialized from the Light Stone, but quickly vanished again, probably due to Black’s inability to support it. It later fully materialized when Black finally came to terms with his 'Truth'.