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As TVLine notes, the press release for "Tiny" does have someone listed as Jack: actress Cassidy Freeman.
That's right: it appears Jack is a woman in OUAT-verse!
We don't know much else about this character, but TVLine reports that show create Adam Horowitz says he's "very, very excited" about this episode, and is happy "to offer a continuation of how [the giant] fits into the overarching mythology of our show."
Catch the next episode of Once Upon a Time on Sunday, February 10, 2013 at 8 p.m. ET/PT on ABC.