Iwako was Medama-Oyaji's wife and Kitarou's mother. She died while pregnant with him, but Kitarou managed to claw his way out of her grave anyway—a pretty metal introduction to the series, honestly.
Most versions of the story just call her "Kitarou's Mother" rather than using her actual name, Iwako. She's defined almost entirely by that relationship and her tragic death, which sets up the whole premise of why Kitarou exists as he does.
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