Reshiram is a wild dragon/fire-type Pokémon who appeared a many times during the anime.
Reshiram first appeared in a flashback when a mysterious man named N first encountered it at his burning castle and flew away. In the episode Secrets From Out of the Fog!, it was revealed that the reason why N encountered Reshiram was because N was being used by the Team Plasma leader, Ghetsis to summon it during a ceremony with the Seven Sages. Reshiram was filled with so much rage and anger that it literally destroyed N's Castle and flew away before going back to sleep in its Light Stone form. Two years later, the Light Stone was recovered by Cedric Juniper from the White Ruins along with Ash and his friends. But before they can research more on the it, N snatched the Light Stone from them to prevent Team Plasma from awakening it. When Ash and N ended up getting trapped under the ruins, N reveals to Ash that he wanted to see why Reshiram was filled with so much anger when it destroyed his castle and he wanted to prove Anthea and Concordia that they are wrong about people being cruel to Pokémon. Meanwhile, Team Plasma attacked the ruins and took Cedric Juniper, Looker, Iris, and Cilan hostage. Ash and N managed to dig themselves out and join in the battle. However, during the battle Colress took control of Pikachu and N was forced to surrender the Light Stone in exchange for Ash's life. After that, Colress freed Pikachu from its control and N was taken prisoner as Ghetsis begin begins the awakening ritual of Reshiram. As the awakening ceremony processes, Anthea and Concordia stepped in and tried to help N and the others stop Team Plasma, but their Pokémon ended up getting into serious trouble when Colress put them under his control. Ghetsis managed to awaken Reshiram from its Light Stone form and Colress took control of it. Once Resiram was being controlled, Ghetsis commanded it to kill everyone at the ruins, turning its full power on its enemies and followers. But when Colress took control of Pikachu once again, Pikachu shocked it and Reshiram began to regain consciousness. Pikachu and Reshiram destroyed Colress' Pokémon control device and all of the Pokémon were freed. But it was filled with so much rage that it continued to go on a rampage. N managed to calm it down and convinces it that not all humans are evil. N tells it that people and Pokémon can live peacefully together as long as it understands the feelings on how humans truly care about it deep within their hearts and it will always be there. Ghetsis tells Reshiram to not listen to N, but Ash and his friends stopped them under Looker's orders. Before Reshiram flew away, N gave a brief conversation with it and asked it how did it feel when it saw how everyone, both people and Pokémon alike, battled against Team Plasma and saved it from being controlled by them. Reshiram looked at N in silence and N thanked it as he understood its answer. N asked it if it could believe in humans as they believe in Pokémon. The world of truth that Reshiram wants and the ideal world for humans. N tells Reshiram that just like it, he will keep fighting for the ideal world that everyone can live in. Reshiram accepted N's request and flew away peacefully into the sky.