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In "Snow White and the Huntsman," Princess Snow White is played by Kristen Stewart and Greta (her name is changed to Rose in the novelization) by Lily Cole.
Greta is a village girl who's making her way to Duke Hammond's castle when she's captured by Finn, the brother of Queen Ravenna, Snow's stepmother. Greta is taken prisoner and is placed in the same tower as Snow White.
When Snow White asks who Greta is, the woman tells her she was trying to reach Duke Hammond. Upon learning that her late father's ally and his son, William, are alive, Snow asks if they still fight in her father, the King's, name. By doing this, Snow White inadvertently reveals to Greta that she's the Princess.
This shocks Greta, as she was told, as was everyone else, that the Princess was killed the same night as her father was. But clearly that isn't true. This, of course, sparks hope in Greta; likewise, the news that the Duke and his son are still alive - and are continuing to fight for her father's cause - sparks hope in Snow White.
Soon after, Ravenna sends for Greta and drains her youth from her. Snow White finds this out upon her escape when she tries to free Greta from her cell, but Greta urges the Princess to flee without her. This is what first motivates the Princess to vanquish her stepmother.
Later, after Ravenna's defeat, Snow White is coronated, and not only is Greta present at the ceremony, but her youth has been restored to her.