The real Cosette died as a baby.
How she died was also partly the Duke being a shitty person.
Keira's mother and her family would harass Cosette's mother whom they saw as a threat. I think its hinted the Duke had feelings for Cosette's mother UNLIKE Keira's.
This is why he seems to treat "Cosette" better and warmly in the first life than he did Keira.
The other wife knew Keira's mother family wanted to harm her and would plead with the Duke for protection. He never intervened or made any effort to take the threat seriously for political reasons.
She becomes pregnant and this when tragedy hits.
Eventually Keira's grandfather hires assassins to target her and Cosette's mother sensing her baby is dying makes a contract with a demon in desperation and despair to save her baby's life in exchange for hers.
Cosette's mother dies and so does real cosette. But the demon enters the dead baby's body and lives in it. This is who the current "Cosette" is.
Last time i said that the mc’s father only turned back time for himself but I didn’t know the exact reason and im here to tell u now. The reason he did it is because after mc died the country kinda got ruined and they blamed kiera’s father for killing the spirit user aka her so he decided to turn time just because he didnt wanna be blamed for her death anymore even tho he executed her KNOWING she was his daughter but he didnt wanna “tarnish” the family’s honor…if she forgives him im out