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The future fiancee (as I don't remember from the bovel if there will even be engagement) has nothing to do with giosas. She in fact is not bad and needs help. The owner of elgiosa is a little girl, they will go to save her soon (in two episodes at most).
Major spoiler
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The fiance is already married and has a daughter. She probably lived happily with her husband and daughter in the 1st timeline. Yurien was never engaged to anyone. The whole reason the engagement is popping up rn is because what was supposed to happen didn't happen i.e. the demon sword's massacre. Butterfly effect if you will.
I'm calling it, the giosa discovered after it's owner dies is the fiancee's giosa, so in the 1st timeline she was supposed to die