“I, who forged the sacred blade... I, who committed the great sin... I, who wove the divine colors... I call on you now, Seal of Flames!”
—The Rainbow Sage empowering the Yato
The Rainbow Sage (虹の賢者 Niji no Kenja) is a non-playable character in Fire Emblem Fates.
Thousands of years before the story of Fates, the First Dragons waged war on each other. To gain an advantage, the dragon known as the Rainbow Sage recruited humanity to fight alongside them. Doing so he forged the Yato, Fujin Yumi, Raijinto, Brynhildr, and Siegfried, but their creation forced the humans into endless wars. As a punishment for his sin, the Rainbow Sage could not die until he could absolve it. During the present he resides in Notre Sagesse and reveals the true power of the Yato to the Avatar.
After the Avatar and their party arrives at Notre Sagesse, the Avatar flags an old man down (the Rainbow Sage) and asks them where they can find the Rainbow Sage to gain power that Xander obtained. He points them up a nearby mountain, stating that the Rainbow Sage resides in the Sevenfold Sanctuary. He warns them that many have gone up, only to never return back down. He tells the Avatar that only Sumeragi, Garon, an unknown knight and Xander have survived the trial. After the Avatar battles through the many shades in the building, they open a door at the final building and end up back at Notre Sagesse. The Rainbow Sage laughs at the Avatar, telling them that he is the Rainbow Sage. They ask for power, but he says that they have already obtained it, similar what Garon, Sumeragi and Xander had done.
After the Avatar asks him about their Yato and the Seal of Flames ritual, he is impressed that the Avatar knows about it. To perform the ritual, he asks the Avatar to bring their Yato while he also calls for Takumi to bring his Fujin Yumi. He performs the ritual and gives the Noble Yato to the Avatar. Before they leave, the Rainbow Sage gives them a paper with a shortcut to Windmire through the Burning Falls.
Although his death is not shown in-game on this route, it is implied that he died shortly after the war between Hoshido and Nohr ended. This is shown just before the end of Chapter 19, as he decides to watch the outcome of the war first.
If the Avatar sides with Nohr they seek out the Rainbow Sage for power and advice. Hinoka and her retainers are guarding the way to the Rainbow Sage. After defeating her group, the Avatar's group meets with the Rainbow Sage. After the Avatar makes their request for power, he notes that the Avatar had already passed his test and been blessed during their battle with the Hoshidan forces. He then tells the Avatar that the sword will transform after meeting with a hero of Nohr. Soon after, though, Iago appears as an illusion with orders from Garon to kill the Sage, in order to prevent anyone else from gaining his power. Though the Avatar is distressed, the Rainbow Sage opts to instead take his own life, to spare them the pain.
After the Avatar 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. After telling the Avatar the story of his past, he finally is able to die as he fulfilled in his role of granting humanity a chance at true peace.