Reshiram is a dual-type Dragon/Fire legendary Pokémon, and serves as a villain in some of its appearances.
Reshiram serves as N's Legendary Pokémon. It first appears at Dragonspiral Tower, where N resurrects it using the Light Stone (which is implied to be its remains). Reshiram appears again inside N's Castle, where it must be fought in a final battle against N.
Pokémon White: The Movie
Sometime during the three years he spent traveling the world prior to the events of the film, Damon happened to overhear Reshiram calling to him while he observed the ruined Kingdom of the Vale alongside Mannes. Together, the two of them went down into the caves inside the Sword of the Vale, where they became separated. At the bottom of the cave Damon discovered the Dragon Stone containing Reshiram and reawakened it.
With Reshiram's help, Damon traveled to the villages of the People of the Vale and managed to convince them to support him. Its first appearance in the film comes when it helps Damon stop a giant tornado in the desert along with a herd of stampeding Bouffalant. It later appears in a flashback as Damon tells the story of his travels.
When he raises the Sword of the Vale and comes to be opposed by Ash, Cilan, Iris, Carlita and Juanita, Damon summons Reshiram to aid him. It battles Juanita's Golurk, which Reshiram easily defeats.
The second time it fights Golurk, it is interrupted by Zekrom, which Ash has discovered and allied with. The two dragons battle after Zekrom drops Ash at the castle, ending in Reshiram's defeat. Zekrom and Reshiram then realize that Damon's actions have unleashed a corrupted Dragon Force that threatens to destroy the world, and both dragons blows away the clouds to reveal this fact to Damon.
Pokemon anime: Episode N
Reshiram appears with a major role in the Best Wishes! Season 2: Episode N arc. It was first seen in N's flashback in The Name's N!. N had a flashback of his youth, a moment when he had been at his castle, which, along with its surroundings, had been on fire at the moment. He saw Reshiram appearing before him, and kept looking at it as it flew away. It was later seen in another flashback in Secrets From Out of the Fog!, rebelling against Ghetsis and Team Plasma by burning down N's Castle during a ceremony involving N, Ghetsis and the Seven Sages.
It was seen in fantasies and as a statue in Ash and N: A Clash of Ideals!. In What Lies Beyond Truth and Ideals!, Ghetsis summoned it using the Light Stone and Colress used his machine to take control of it. Ghetsis used it to attack Ash, N and the others when they tried to resist him, but it was ultimately freed once Ash's Pikachu destroyed Colress's machine. N was then able to convince it that not all humans were evil, and it departed peacefully.
In the Pokémon RéBURST manga
Fraud, a leader of the villainous Great Gavel has a Reshiram in which he can use as a Burst form.