It was an anime series that I remember first finding at Video Warehouse rental stores both near me and in NYC around 1998-1999.
the protagonist was, i believe, a girl with i think a female ghost acting as an alternate personality. they would swap when a fight was needed and would eventually turn back. it was set in modern era Japan. The anime was dubbed but I can't remember the name nor any character names whatsoever.
sorry for it being such a vague explanation but I would love for my wife to watch this as I think it'd be a great show for her.