"The One-Armed, Horned Sennin"
Introduced in the official comic 東方茨歌仙 (Touhou Ibarakasen) ～ Wild and Horned Hermit, serialized in Chara☆Mel Febri. An enigmatic pink haired, red-eyed "pilgrim" with her right arm wrapped in bandages whose search for the "Kappa Arm" led her into the Hakurei Shrine. It is soon revealed that there is no arm inside her bandages, which are instead filled with a mysterious magical smoke.
Due to the shackle on her left arm, her suspicious hair buns, and her title implying that the existence of horns, there is rampant speculation among fans that Kasen is an oni, specifically Ibaraki Douji, one of the Big Four. Some of Kasen's actions, such as her avoidance of Ibuki Suika, also contribute to the speculation.
Since both characters are hermits, Kasen is often associated with Kaku Seiga, though usually on an antagonistic basis since Seiga is a "wicked hermit".
Wild and Horned Hermit 35 revealed she is one of the Sages of Gensoukyou alongside Yukari.
Wild and Horned Hermit 49 would not only introduce her severed arm given semi-physical form, Ibaraki Douji's Arm, but Kasen herself would become one with her arm and fully become an oni once more, revealing herself to be one of the Big Four, Ibaraki Douji.
Playable in Touhou 14.5 ~ Urban Legend in Limbo.