I don’t think it is Kathryn. Though the Siren can become any woman she wants to. There are photos from this episode. Here is one of the siren.
And here is a link to the rest of the pictures from this episode.
http://www.spoilertv.co.uk/images/once-upon-a-time/season-1/promotional-episode-photos/Episode%201.13%20-%20What%20Happened%20to%20Frederick/
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