Andy's Reshiram

Reshiram is a Pokemon that Andy captured during the Unova Arc in N's Castle.

Unova Arc
Andy captured Reshiram during the near end of the Unova Arc, after awakening it from the Light Stone given to him by Lenora. After Andy's battle with N, he returned to Nuvema Town and decided it would be better off if he released Reshiram instead of keeping it, but Tristan suggested that instead that Reshiram's release should only be temporaily.

Kanto/Unova Arc
Since Reshiram can be called upon at any time, it was called upon so Andy can once again battle N but this time with the two being friends.

Trivia

 * Reshiram is Andy's first Legendary Pokemon.