Well, she is literally just that; Kasen's severed right arm taking the form of an oni on its own. Manga Reader
If you'd like to know more, give it a read up until the final chapter. Those were the only times the arm ever appeared in the canon.
Ahhh, so the arm took up a form. So, just wanna make sure: did the arm take a near exact form of Kasen? I’m just asking because I am not familiar with the Touhou Project lore and characters.
Manga Reader
If you'd like to know more, give it a read up until the final chapter. Those were the only times the arm ever appeared in the canon.