The Light Stone made its main series debut in Ash and N: A Clash of Ideals!, where it was found in the White Ruins by Cedric Juniper. N soon snatched it away, claiming that he wanted to ask Reshiram why it had attacked Team Plasma's castle years before. Ash tried to chase after him, but got stuck in a sinkhole. N tried to help him, but ended up falling down the same hole with Ash. Using Pikachu, Pignite, Krookodile, and Charizard, Ash was able to dig them a way back to the surface, where Team Plasma had started an attack on the White Ruins, intending to get the Light Stone for themselves. When Colress used his Pokémon Control Device on Pikachu, forcing it attack Ash and his friends, N surrendered the Light Stone to Team Plasma in exchange for Ash's safety. Ghetsis then proceeded to conduct a ritual to awaken Reshiram, successfully restoring the Vast White Pokémon to its true form.
In the manga
In the Pokémon Adventures manga
The Light Stone in Pokémon Adventures
The Light Stone in Pocket Monsters BW: The Heroes of Fire and Thunder
The Light Stone and Dark Stone initially resided at the Desert Resort, where they were found and excavated by Clay and his mining team in Lights, Camera...Action!. After investigating the stones, he felt worried and called Lenora to come and investigate them with her expertise on ancient artifacts. Lenora took both stones to her museum, but only made the Dark Stone discovery public so that the Light Stone could be kept a secret. Later, Lenora called Drayden to help her investigate the stones, which helped them realize that they were actually Zekrom and Reshiram.
After the Dark Stone was stolen by Team Plasma, Brycen, having noticed the Light Stone rolling out of its safe towards Black, and thus acknowledging him as the Hero of Truth, agreed with Drayden to entrust the stone to Black, which he did in A Cold Reception, after being beaten by Black in a Gym battle. In True Friends, during the Pokémon League tournament, Black's rage over Team Plasma having convinced Cheren to side with their beliefs caused the Light Stone to be awakened into Reshiram. He then proceeded to use Reshiram to battle N's Zekrom, eventually emerging victorious. After the battle was over and Team Plasma had been defeated, Reshiram started reverting into its stone form once again. Ghetsis, wanting to get revenge on Black for foiling his plans, used Colress's Beheeyem to push Black closer to the forming stone, causing him to get sucked into it as well. The Light Stone then floated away before White could grab it.
In Flying Ship, the Light Stone was seen flying towards the Unova region alongside N and Zekrom, having reacted to the awakening of Kyurem. Fennel later sent White to search for the stone at the Entralink, where she was able to find it. N, having followed the stone, told White that the stone had landed to the Entralink on purpose for her to find. In PS544, the inside of the Light Stone was revealed to be connected to the Pokémon Dream World. After White had been briefly sucked into the Stone and reunited with Black, Reshiram emerged from the Stone, freeing both Black and White from it in the process.
In the Pocket Monsters BW: The Heroes of Fire and Thunder manga
The Light Stone appeared in Get the Legendary Pokémon!, where Shin and Alder found it at the Relic Castle. After an apparent betrayal by Alder, which was soon revealed to be merely a test to see if Shin was worthy for the Light Stone to accept him as the Hero of Fire, the stone was revived into Reshiram. Shin then flew off with Reshiram to confront N and Zekrom at N's Castle.