—The Rainbow Sage empowering the Yato
The Rainbow Sage 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 Fujin Yumi, Raijinto, Brynhildr, and Siegfried for his human champions to wield. He also created the Yato, the strongest of the weapons he created as it draws power from the other four. The strength of these weapons plunged humans into an endless cycle of 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 Corrin.
After Corrin and their party arrives at Notre Sagesse, Corrin flags an old man down 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 Corrin that only Sumeragi, Garon, an unknown knight and Xander have survived the trial. After Corrin 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 Corrin, 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 Corrin asks him about their Yato and the Seal of Flames ritual, he is impressed that Corrin knows about it. To perform the ritual, he asks Corrin 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 Corrin. 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 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. Corrin also intends to speak with the Rainbow Sage so they can receive power. However, upon arriving at Notre Sagesse, they learn that the Rainbow Sage was taken by the Hoshidan army into the Sevenfold Sanctuary to serve as his new "bodyguards". Corrin and their army defeat the Hoshidan army stationed in the sanctuary, who were led by Hinoka.
Upon routing the Hoshidan army, Corrin finds the Rainbow Sage and asks to be blessed with his power. He laughs at Corrin, as they obtained the power while fighting his "bodyguards". The Rainbow Sage inspects his former creation, the Yato. Since one of Nohr's heroes is not with them, the Rainbow Sage prematurely starts the Seal of Flames ritual, telling Corrin that when they seek the aid of another Nohrian hero, and that the Yato will be transformed into the Grim Yato when that happens. He also knows of its final form, but only when they meet their fate head-on.
Iago soon greets Corrin and relays to 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. 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 begins dying inexplicably, and reveals to them that he is the dragon that they were looking for, having known of Izana's fortune. 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.