Kuen

was a former shrine maiden who sealed Sōma four-hundred years ago.

Background
Four-hundred years ago, Kuen confronted Sōma. She managed to seal the demon inside of her body. However, with the rapidly draining memories in her, Kuen failed to slice off both emerging horns to complete the seal. Instead, it was sealed in a nearby rock with a portion of it staying within Kuen. Sōma's curse left Kuen forever purged of her memories, leading her to step down as the shrine maiden and to bear twin children which the curse was passed down to her descendants.