Iwako (岩子) was the wife of Medama-Oyaji and the mother of Kitarou who died while she was still pregnant with Kitarō, but he climbed out of her grave.

In most adaptations, she is never referred to by name, only ever as Kitarou's Mother (鬼太郎の母 Kitarou no Haha).

(Source: GeGeGe no Kitarou Wiki)