There's a zerochan image suggesting that it's Irisviel:
However, it credits a Pixiv user so chances are that it ain't official. Nevertheless, there are some bases for such an assumption:
Irisviel has a connection to Saber (being the stand-in Master during Fate/Zero)
the arm looks feminine
Despite Logan M's comment Fate/Zero characters do appear in the ...
This is an interesting question, but without an official source to confirm the answers, they can only remain as speculations.
If you pay attention to the forearm of the hidden character, it is not covered in sleeves, nor is it decorated with anything, like a wrist band, so Kiritsugu is out. Saber's friendly gesture towards the unknown person shows a ...