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Ouka Furude (古手 桜花)Furude Ouka is the daughter of Hanyuu and Furude Riku. Though reluctant, she is the one who performed the Watanagashi ceremony on Hanyuu because Hanyuu felt that if she died, Hinamizawa would calm. She also left the chip on Hanyuu's horn when she tried to kill her. Ouka is the one who most likely started the tale of Oyashiro-sama's curse, possibly to scare the people in revenge for her mother's death.
Rika is the reincarnation of Ouka, which explains why she can only see Hanyuu. The two also look very similar, except with different colour hair.
(Source: Higurashi No Naku Koro Ni Wiki)