User blog comment:Inept Wiki User/Lostbelt No. 2: Götterdämmerung, Pre-Release Information and Speculation/@comment-30693328-20180718170636/@comment-4802891-20180718193206

Yep, Pseudo-Servant Sitonia (or whatever is the orthograph for it, can't find and English source), a hero from Ainu mythology, a Dragon (Serpent) Slayer, in the body of Illya.

She doesn't seem to be playable just yet, like Muramasa. However, this makes it clearer and clearer that the 2004 HGW of Fuyuki, the one shown in FGO, is a starting point for much more than the Goetia arc!