This is the stage 5 midboss from Mystic Square. I don't know how to tag this. Extra?

Are you sure it's not Elis from the first game? Kinda looks like her.

For your observation:

Elis

Mystic Square stage 5 midboss

No bat wings, no star facial mark, and those weird spiral things off the hip of her dress. It's the midboss.

Additionally, title on Pixiv includes 怪綺談五面中ボス, which roughly translates to "Kaikidan (Mystic Square) fifth midboss".

I love how in Touhou everybody gets fanart. No exceptions.