I don't think she's a witch. If we are to follow the anti-fantasy credo, the sole fact that people talked to her at the mansion (see
this screenshot) means that she's a human being like any other.
Again, as I said earlier, if we were to consider her a witch, we'd be either admitting the existance of witches (which would lead to Battler's final defeat) or we'd be admitting she's an other fictionary/alternate "identity" in the same vein of Ronove, Gaap, Virgilia, the goat servants, etc.
So here's two options we have regarding this character:
- She's a "real" character, meaning that she's a new one who's introduced on the island at present. A new piece on the chessboard, raising the counter to 18 people (if we are to assume no one else is already dead before the family conference starts). That, or she's someone from the past (but I am not too sure of it).
- Or, she's a "Beatrice", not in the sense that she's a witch (as in a magical being), but in the sense that she's an "alternate" identity of someone among the people present on the island, much like we speculated that the Beato that we've seen in the previous episodes was a role personificated by someone (or like when we speculated that Ronove = Genji, Kumasawa = Virgilia, Gaap = Shannon, etc). And if we had to consider this latter theory, who could be the person taking on this role? If we base this on the painting and the fact the the girl shows no family crest on her clothing (nor does she carry things that hint that she's linked with the Ushiromiya family), there aren't a lot of options left among the 17 people. Maybe Natsuhi? She isn't allowed to carry the crest, afterall.
As for the hand at the bottom right, it could be either be Battler's hand or "Beatrice's" (meaning the person who was impersonating that "role" in the previous episodes). Although I have to say that hand looks a bit too big and manly for a woman's hand, which is why I'd rather say it's Battler's, or maybe Kinzo's.