* "Jill Pole" is played by Millie Davis. She portrays a strong-willed, adventurous girl who is part of the quest to find Prince Rilian. This character is closer to the original book's Jill, but her name is not explicitly stated in the film.
* "Polly" is played by Ellie Kendrick. She portrays a kind and timid girl who is initially reluctant to join the quest but ultimately proves invaluable. While not explicitly "Jill", this character embodies many of Jill's qualities from the book.
So, technically, there isn't one actor playing "Jill" in the film, but rather two actresses who share aspects of the character.