How is Safie a feminist character?
Safie is the young foreign woman who comes to live with the DeLacey family at the cabin where the creature is hiding. She is a feminist character because she travels virtually alone (with only one chaperone who does end up dying on the journey leaving Safie at the mercy of locals to lead her to her love) to the man she loves, Felix. She defies her father's wishes and the mores of the time period regarding young women (they were expected to do as they were told, first by their fathers and later by their husbands. It was not appropriate for women to travel alone and unprotected as she has done).
After she has arrived safely at the cabin with the DeLacey family, she strives to learn their language through lessons from which the creatures also benefits, unbeknownst to the cottagers. She is a strong, independent spirit who works hard to reach the goals she has set for herself and lets little trip her up on the way to her destination.
I would say these things point to a very "women's rights" sort of feminist character, although I confess I never thought of her in this light before your question appeared on my computer screen. Thanks for making me think today!