variant sanae
Wicked and extremely deadly doppelgänger of Kochiya Sanae from Touhou Igyoukyo.
Her form varies from appearance to appearance (unclear if she can change it or if it's due to all of her appearances being possibly intangible "hallucinations" or visions), but it's generally similar to "prime" Sanae down to exact clothing, except with extremely long and lanky limbs, especially her neck, and sometimes with tentacles coming from her back, giving her a similar appearance to Slender Man in certain conditions. Her jaw is constantly open and often very wide, as if unhinged, and it has multiple rows of teeth. Her eye sockets are generally obscured by darkness, but appear to contain multiple eyes at once.
She has so far only appeared in "illusions" or perhaps pocket dimensions with her victims under her curse. The recurring themes of the things seen in them appear to be a reference to Sanae's backstory in Koishi Komeiji's Heart-Throbbing Adventure, just with the situation reversed so the curse victim is the subject of the torment.