Given this reply to a comment on the artist's twitter post of this image (the first reply itself is deleted, but given the image the artist responded with, we can reasonably surmise that they were also wondering who is being portrayed here), I think it's just Reimu in this piece, based on some very old ZUN art. Might be appropriate to give it the PC-98 Reimu tag, though I think the image itself is actually from the period between Mystic Square and EOSD, so maybe the design differences are just representative of ZUN playing around with her presentation a bit before he settled on what we see in EOSD.
It should be noted that most of the information here was gathered from various old forum posts, wiki pages, and blogs, often in a language I'm not even close to fluent in, so do take the details with perhaps a few grains of salt.