Princess Kaguya Ootsutsuki is best known as the mother of Hagoromo and Hamura Ootsutsuki, and basically the entire reason chakra exists. She ate the fruit of the Shinju and became the source of all chakra—a pretty huge deal in the series' cosmology. Through her sons, she's essentially the ancestor of every major clan and ninja ability that matters in the story.
Her backstory kicks in pretty late in the manga, first appearing in Chapter 670, which is when things get genuinely cosmic in scope.
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