Kuen

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

Background
Four-hundred years ago, Kuen confronted Sōma. Sōma almost managed to possess Kuen, but Kuen sealed it in a rock. However, despite sealing Sōma completely, a portion of Sōma stayed within Kuen, leading her to step down as the shrine maiden and to bear twin children which results in a curse that was passed down to her descendants.