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 Mokou was the daughter of a nobleman (assumed to be Fujiwara no Fuhito), one of Kaguya's suitors during her initial exile on Earth. When he was humiliated by Kaguya's impossible request, Mokou came to hate her. She wanted to take revenge, but Kaguya disappeared before she was able, so she consoled herself by stealing the Hourai Elixir that Eirin had bribed the other humans with, and drinking it to become immortal.

Source from Wikipeida