The younger twin sister of our water princess protagonist, and the surrogate princess of the kingdom of Flame.
She's introduced as an antagonist, does some actually heinous stuff for a 12-year-old (including at least 1 count of murder), and looking back, it's honestly insane that this was marketed to tweenagers. However, it's noted that she was both brainwashed and just jealous, and the black star on her neck eventually fades to become a clear blue, allowing her control of clean air. It's nice that Paseri forgives her, but still holds Mimosa accountable for some of the stuff that she's done.
"What is this series? Why is the princess called 'Parsley' of all things?" Great question. Unfortunately, this was only ever available in Chinese and Japanese, so the translations were much more graceful. The author, Yoko Kurahashi, also wrote Koe no Katachi, which did get translated into English and an animated movie, I think. My pipe dream is that PaseDen gets adapted into an anime one day.
Icon art by me, as the illustrator has a strict no re-use/re-upload clause.