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REAL CASOTTE'S SAD STORY SPOILER
The family of Keira's mom kicked out Cossette's mom from the duchy, who was the first wife of the duke, coz the family of Keira's mom is powerful. they kicked out/assasinated the 1st wife and her child, who is Cossette and she died. so Cossette was supposed to be real but since the 1st wife wanted revenge coz she was full of hatred about what happened to her, she made a deal with the devil to revive Cossette. so the one who's in Cossette's body is not the real daughter now, but the devil. however, the devil wanted to ruin and end the world by killing Keira who is the saintess. Keira's mom is nice and a good mother but her family is powerful and kinda greedy and she was also a victim. On the other hand, the Grand Duke didn't do anything, he neglected his own family because he wanted to stay neutral in society thus ignore both his wives. even though he got feelings for his 1st wife (Cossette's mom), he ignored her pleas for help. same with 2nd wife (Keira's mom) who he ignored, hated because of her powerful family and sees her only as a breeder. he is the main reason of all the sufferings because he can't protect his own family. he failed as a husband, father and as the head of the family.
THEORIES
So here's a theory I came up with on why the Duke ignored Kiera:
•So just like how Claude (WMMAP) chose to ignore Athanasia because of his feelings towards her mother, I think that the Duke decided to ignore Kiera because of his feelings towards her mum
•But unlike Claude who ignored the original Athanasia because he didn't like that fact that his beloved (her mum) chose her over him, I think the Duke chose to ignore Kiera because he didn't think highly of her birth mother (the second wife)
•Based on the spoilers provided, I think the MC's mum wasn't a good person ----- in fact, I think she was one of the people who was involved in organising the assassination (wouldn't be surprised if the MC's mum was the one who brought up the idea of assassinating the first wife to her family)
•Going off of this theory, I believe that the Duke ----- who doesn't think highly of the MC's mother ---- probably went by the motto: "The apple doesn't fall far from the tree" and thus chose to take out his dislike towards the mother on his innocent daughter (the MC).
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My thoughts and theories:
•The MC's mother and her family were probably crooks
•The Duke who knew the MC's mother and her family were crooks, didn't think highly of them and regarded them as nuisances
•The MC's mum was probably a jealous woman, like a REALLY jealous woman. Jealous enough to bully, harass and eventually murdering the first wife
•The MC's mum was probably either crazy and ambitious or smart, calculative as well as ambitious.
•The Duke probably was naturally nicer to Cosette because she reminded him of his first wife (who he actually loved)
•The Duke could also be treating her better as a means to make amends for not taking action when his first wife was getting bullied and eventually murdered
•The Duke might also be playing the "Sure...kill them if you want, but just because you've killed them and their kids does not mean I'll acknowledge you let alone your children" card towards the mum
•Still going of my theory that the MC's mum might've been crazy and jealous, she might've killed the first wife with the hope that the Duke would acknowledge her and favour her now that the "nuisance" (the first wife) was out of the picture. But not only did killing off the first wife make the Duke more disgusted and revolted by her personality, but it also made him distance himself even more from her as well as her offspring.
-I_Am_One_Hell_Of_A_Butler (Well-Known Member of Forum Novel)
EXTRA INFO
Cosette is the real daughter of the duke too but she died when Keira Parvis was born (actually, they were born at the same time But Keira's mom assassinate the child) and the devil (which is bijj) possessed Cosette's dead body through Cosette's mother wishes or she make a deal to the devil. That's why devil said to Keira in the past that "You're actually the real one" cause the devil want to conquer the entire kingdom and in order to do that, the devil needed the power of the duke. To my point of view, The Duke is the actual villain of the story, ignoring his first loved wife plea and ignoring the second wife offspring.
I feel bad for the Dead Baby Cosette, she was born with love but she was being assassinated the moment she was born. She can't even experience a mother's love, a mother's touch and a mother's voice. And especially for the first wife, she married the man she love but that man which is the duke couldn't help his only love just because Keira's mom is powerful. How devastating it is for the first wife, she married the man that cause her death and couldn't embrace her one and only child any longer. They were being assassinated that is being considered to be nuisance by Keira's mom. They are just an innocent mother and daughter, they can't do anything but wish to survive and live together just the two of them. But what waiting for them is at the death's door. That's why the first wife wanted to revive Baby Cosette's body in order she could live and experience happiness. But it's not the real Cosette is being revived. Instead of being revived, the devil used the death body of baby Cosette in order to have the power of the Duke and use it to destroy the world.
But I hope Baby Cosette is happy and well in the heaven or in the next life. And I also hope that the mother and daughter are now resting in peace together in the field of garden.
May You Rest in Peace, The Mother of Cosette and Baby Cosette
MAJOR SPOILERS
Keira went on adventures and forgive his father which is the duke but both Keira's little brother and the duke went on a separate ways. There is no actually an ML of this manhwa.

correction to ur spoiler
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so it wasn't cossette mom who made deal with the devil but her brother who after seeing his sister and her son dead swore revenge on the duke and the world
cossette the actual human was a male and was never a female but the demon when possessing him changed his gender to suit his needs(since the family demand a saintness)
the mc mother was an ok person not the nicest like her family bullied the first wife she didn't engage in the bullying but ignored it like the duke since she is supposed to be her father asset and she can't go against him
the duke loved his first wife and blamed the second wife (who he already hated) for her death despite her being just a bystander like him (and she lacks any power to go against her father anyway unlike him who could have stopped the whole thing with his power )
the mc never forgave her father for how he never cared for her as a daughter and mainly for her role to save the world and his ass
the brother joined the sister and left him alone to regret his decision
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