Ibaraki Douji? Who's that?

The youkai Kasen is likely to be based on. Ibaraki Douji was an oni who had lost an arm fighting Watanabe no Tsuna, and liked to hide in the mountains. Both characteristics fit Kasen, and add to that the title 'horned hermit' and the shackles (a typical oni feature) and the dots connect.

Aside from the name, the arm, the shackle and her title implying she has horns, the Wild And Horned Hermit manga HEAVILY implies Kasen is an oni.

Like, at one point Suika shows up and Kasen immediately runs away and hides, going all: "Shit, it's Suika! She'll blow my cover!"

Kasen and her arm.

Hilarious in hindsight material? The artist even got the horn correct somewhat.