Reshiram first appeared in The Name's N! in a flashback N had during his time in his castle. It flew away when the castle was set on fire.
It reappeared in Secrets From Out of the Fog! in another flashback, following N's admission that he was once a member of Team Plasma. In this flashback, it was revealed that Reshiram was responsible for burning the castle.
Reshiram reappeared in a flashback in Ash and N: A Clash of Ideals!.
Reshiram was revived from the Light Stone in What Lies Beyond Truth and Ideals!. Following the revival, Colress zapped it with his machine and Ghetsis took control of it. Because of Pikachu and Reshiram rebelling against the machine, Colress and Ghetsis lost control of Reshiram. Before Reshiram flies off, N stopped it and said that he always wanted to meet it. N asked Reshiram how it felt when it saw everyone battle against evil in the ruins, and Reshiram silently responded to him by looking at him.
N also asked if it could believe in humans as they believe in Pokémon. He then told 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.