gensokyo's first hakurei miko
The Miko of the Hakurei Shrine from the time when Gensokyo was first formed. Revealed in the end of Foul Detective Satori to be the one who sealed Miyadeguchi Mizuchi away.
Note that she is not the direct canon equivalent of Sendai Hakurei no Miko, a character from M.U.G.E.N depicted as Reimu's immediate predecessor. This Hakurei comes from long before that time.