Chasing after Erw in circles, it was the imouto-san of the Earl-sama who owned this mansion.
Apparently she was an unlucky and remained unmarried in her lifetime.
Because of that lingering affection, it seems she stayed behind becoming a ghost in this mansion.
If it was a male holy magician who came to perform a purification, she rushed at him in arousal, and if it was a female holy magician she ended up enduring the purification with willpower and obstinacy.