SPOILERS ABOUT VILLAIN . . . . . . . . . . . . . . Cosette "technically" is the Duke's dau...

SPOILERS ABOUT VILLAIN
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Cosette "technically" is the Duke's daughter as well.

The issue started with the the Duke and his first wife. The first wife loved the duke, however she was bullied and harassed by staff and other nobles. Partly because of her low status but also because it was all orchestrated by the MC's grandfather, who wanted his own own daughter to marry to Duke because of the power and prestige it would bring. The MC's grandfather is the one who fabricated the lie that the first wife was barren. Because of the Duke's shitty neutrality policy, he never defended or protected the first wife. He basically tossed her aside after it was spread out that she was barren and then finally divorced and banished her so her could marry the MC's mom.

The first wife was pregnant when she was sent away and the child that she was pregnant with was Cosette. However, the first wife wanted revenge on the Duke so she made a deal with a demon. As part of the price, the demon took over Cosette's body.

So technically, Cosette's body is the Duke's daughter, but the mind and soul occupying it is a demon.

In the original time line, after the MC is executed, the demon becomes super strong and leads a demon army and destroys the world. In the new time line, the MC manages to stop the demon. She then also purifies Cosette's body and buries it in the family cemetery. She feels really bad for the first wife and her half-sister for being victims of her grandfather's thirst for power and of the duke's indifference.