Yoimiya sports her hair usually up in a bun. She could easily lose the hair ornament and bun to sport longer hair as a different style. (though I could see an argument for HOW long).
That aside for a moment, these are not fischl's eye/hair colour, her eyes are green, Yoimiya's are yellowish. The artist appears to be faithful to the eye colours of the characters for each piece, and there are more to the set to look through. I could be wrong, but I did at least cross reference several things before making a call. I understand that it does look very like Fischl in face and style of clothing/hairpiece, though.
If we had a proper hair ornament or another telltale piece, I could go a different direction with the character, but at the moment I believe it's Yoimiya for purposes of identification.
