User blog comment:Jo SHININGSTAR/Go! Princess Precure OP/@comment-25034020-20150211092542

Yes we have wondered about who the mysterious girl is. There was a blog created about this a day after the first episode premiered. Anyway, onto my thoughts :)

I think, this mysterious woman is the so called Cure Scarlet and will be a compeltely new character (meaning she isn't Yui) and will do either Setsuna/ Ellen or she'll do a Cure Mirage on us, telling us she was a Pretty Cure before the series began but Despear came along and brainwashed her. She becomes Cure Scarlet again after Flora, Mermaid and Twinkle purify her brainwashed soul.