Late reply, but since this won't be a one-off tag, I'll add a brief explanation.
Basically "Miko" (Shrine Maiden of Hakurei Shrine) is a fan-created character for MUGEN, a fan community-created fighting game. She's Reimu's predecessor, and (one theory, now debunked) an attempt to bridge the gap between the old Touhou PC games and the current windows-based games (she's the "Reimu" of the old games, and the current Reimu we know of starting from Embodiment of Scarlet Devil is her daughter).
She doesn't even have an official name, but Miko was the accepted name used for her. Which became awkward when an actual character named Miko was introduced recently.