Oh, so it seems she really wasn't the original Saintess (though I still have my suspicious). But the original one does seems like a cow, and I can't think of a single reason why she would purposely give away her body... perhaps she lost her holy powers (she murdered someone), and looked for a spirit that had holy powers like her? This would also explain why someone else why holy powers appeared - there was no saint, as she has lost them.
