Shu Keigetsu

The titular "court rat" and the maiden from the Shu clan. She was ridiculed by her clan's court ladies for being made the maiden out of pity by the clan's noble consort and makes Reirin the subject of her rage and jealousy. She secretly learned forbidden magic to swap bodies with Reirin to steal her status and reputation but gives up her healthy and strong body in exchange for Reirin's frail body.