The Rainbow Sage is one of the First Dragons who waged war against eachother in the distant past of the world of Fire Emblem Fates. The Rainbow Sage is notably the creator of the weapons wielded by the royal families of Nohr and Hoshido as he planned to rally humanity on his side of the war by giving them these weapons. This only engulfed the entire continent in endless wars that spread to humanity and the Rainbow Sage disappeared from the public eye with the goal of spending the rest of his extremely long life to fix the sin he commited.
Thousands of years before the story of Fates, the First Dragons waged war on one another. To gain the upper hand, the Rainbow Sage reccruited humanity on his side. He then forged Fujin Yumi, Raijinto, Brynhildr, and Siegfried and gave them to his human champions to help them fight the other dragons. As a way to complete the other four weapons, he created the Yato, his most powerful creation as it drew power from the four other legendary weapons. The creation of these weapons backfired however as humanity was soon plunged into an endless cycle of war fueled by the desire to control the weapons and the superiority that the control of one of them gave to an individual.
Horrified by the situation he had created, the Rainbow Sage disapeared from the public eye for the next thousands of years and decided to work to atone for the sin he had committed for the rest of his long life. By the time of the game, the Rainbow Sage resides in Notre Sagesse where he disguise himself as a human sage and test strong warriors in hope of finally meeting one that could save the world.
After Corrin and their party arrives at Notre Sagesse, Corrin encounters an old man and asks them where they can find the Rainbow Sage to gain power that Xander obtained. The old man then points at the nearby mountain and tells him that the Rainbow Sage lives at the Sevenfold Sanctuary on the top of the mountain. The old man also warns Corrin that almost no one ever survived the trial and that only Sumeragi, Garon, an unamed knight and Xander have come back.
After the group battles many enemies during the trial and open the final door, they are sent back before the old man at Notre Sagesse. The old man laugh at them and reveal that he is the Rainbow Sage and that they have succeeded. They ask him for power but he tells them that they have already obtained it. After Corrin asks him about their Yato and the Seal of Flames ritual, the Rainbow Sage calls Takumi and uses his Fujin Yumi to awaken new power in the Yato, which transforms it into the Noble Yato.
If Corrin sides with Nohr they are sent to Notre Sagesse to clear the area of Hoshidans before they could establish a foothold onto which to attack Nohr. Upon arriving to Notre Sagesse, they discover that Ryoma has already been blessed by the Rainbow Sage and that the Hoshidian army has now captured him and that he is protected by a squad led by Hinoka in the Sevenfold Sanctuary. After defeating them, Corrin ask for the Rainbow Sage's blessing but he laugh at them and tells them that by clearing the Sanctuary, they obtained the power they have been searching for.
The Rainbow Sage then notices the Yato and performs the ritual to break the Seal of Flames. He then tells Corrin that the Yato won't improve until he manage to rally one of the legendary weapon wielders to his cause. Iago soon appears and them that Garon has ordered the Rainbow Sage's death to ensure no one else will use his powers. Corrin attempts to argue that Ryoma had already claimed his and that killing the sage would be pointless. However, the Rainbow Sage appears in the middle of their conversation and kills himself in front of Iago to spare Corrin the pain of killing him, claiming that he didn't intend to live for that much longer. Corrin mourns the Sage's death.
After Corrin decides to search Notre Sagesse in hopes of asking the Rainbow Sage where to find a "dragon", the Sage, disguised as an old man, sends them to face his trials before meeting him. After completing the trial, the Sage powers up the Yato, transforming it into the Alpha Yato.
The Sage then begins dying inexplicably, and reveals to them that he is the dragon that they were looking for. He tells them about his past and how he created the legendary weapons for humans, dragging them into the dragons' war and could not die until he atoned for his sins. Due to Corrin's desire to end the war and the threat manipulating it, the Sage is finally able to atone for his sins and dies in peace. Corrin vows to not let the Sage's sacrifice be in vain as they head to the Bottomless Canyon.