The Holy Grail of Eris, Vol. 1 (light novel)

Connie is a young noble lady whose only standout quality is her sincerity. Maybe that's why she loses her fianc and gets framed for a crime all in the span of one night. Just as all hope seems lost, the ghost of an infamous villainess who was executed years ago possesses her and clears her name. With her incomparable charisma and intelligence, the spirit of Scarlet Castiel turns the tables in the blink of an eye. In return, Connie insists that she help uncover the truth surrounding her new benefactor's untimely death and a conspiracy that continues to this very day!