Maybe they're a beastkin fae of the Gemini Tribe? Regarding the other elements noted, I have a few ideas.
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[*]Darkspawn are known to possess any bodies they can get their hands on - animals included. It might actually be a really good strategy for them, because it's away from civilization and harder to control. So your character may find herself on a quest to save her forest from the Corruption, linking her to the Witch in the greater plot. Perhaps she could have a special power that allows her to speak to animals, or take various animal forms.
[*]If your character was isekai'd here, she may not step into her role immediately. I could see her starting life as an adventurer, and maybe growing into her calling as a Friend of the Forest. She may be more interested in searching for ways to cure magical diseases and connect people to each other than in death and killing. So this character might turn into an ambassador of sorts mediating between the various factions.
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Just throwing out a couple ideas to jog your mind.