Her concept and some of her dialogue heavily suggest that she is actually something else or, at least, she wasn't human before becoming a proper hermit.
Then again, Kasen and Seiga are the only examples of hermits actually seen in Touhou.
Seiga, Futo, and Miko all underwent the same traditionally defined shikaisen process, with Futo being the only one to actually be called or identify as one. Shikaisen are a class of enlightened or ascended human, or "Xian", which share numerous similarities with and are the basis of hermits, celestials, and such.
tehSandwich said: Her concept and some of her dialogue heavily suggest that she is actually something else or, at least, she wasn't human before becoming a proper hermit.
There's nothing to say that only a human can become a hermit.