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Christa's flashback, and what Ymir said. And being the heiress to something important. But ultimately thrown out. Ymir notes she's alike. Could be a clue that she's meant to be the Ymir but didn't in the end and was cast away. Maybe there are other "Ymirs" out there, as I said. Though I don't know if we're getting into clones or not.
Then again, she could be the Ymir, but stripped of her 'position', maybe a shell of her former self. But she doesn't seem very unique from what we've been shown thus far. No special hax abilities and even her titan form looks inferior to what Annie and Eren could muster.