Yeul is an enigmatic character from the beginning, she doesn't have much in-game time, as well as little dialogue. If she speaks, she does so cryptically. However, when the story unfolds it turns out she is actually a crucial character.
She is first seen being laid to rest in the sea of Valhalla by Caius Ballad, her death being seemingly Caius' motivation to fight Lightning.
It turns out later in the game, that she is a seeress priestess from the time of the war of transgression, which she foresaw five hundred years ago. Being blessed and cursed with the "Eyes of Etro" that let her see into the future, she transferred her visions to the oracle drive. However, this gift of Etro comes with a major sacrifice: every time she has a vision, her lifespan is shortened.
She dies young just to be reborn, over and over again, to continue foreseeing the future. Trapped in this cruel cycle, her fate is Caius' motivation to destroy time by killing Etro, creating a world without future where Yeul can live free from her curse.
At the end of the game Noel states that Yeul actually chooses to be reborn, so she can be with Caius.
Her full name is Paddra Nsu-Yeul, which means "Yeul of the Nsu clan, from the city of Paddra."
She is voiced by Mariya Ise in the Japanese version, and by Amber Hood in the English version.