
SPOILERS!!!
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So Cosette was the daughter of his previous wife, and like he thought so too. I mean, she looked like him, and was given the name discussed by him and his previous wife. He didn't care for her but rather the reputation of his own family, so by killing off Keira it would look like he wasn't exactly weak and would so no mercy. I don't know anything about hypnosis, but we do know that his story isn't like Claude's from Who Made Me A Princess.

He also let his first wife be mistreated, abused and killed by his own family members...
https://forum.novelupdates.com/threads/actually-i-was-the-real-one-%EC%82%AC%EC%8B%A4%EC%9D%80-%EB%82%B4%EA%B0%80-%EC%A7%84%EC%A7%9C%EC%98%80%EB%8B%A4.113065/

Correction: It was the family of MC's Mother. But he still did nothing.
Now I can understand more why MC was the primary target of the demon.
He still an indecisive weak cowardly asshole.
https://forum.novelupdates.com/threads/actually-i-was-the-real-one-%EC%82%AC%EC%8B%A4%EC%9D%80-%EB%82%B4%EA%B0%80-%EC%A7%84%EC%A7%9C%EC%98%80%EB%8B%A4.113065/page-6
So he actually cared about her? What Happ in previous life then? Was he hypnotized or something? That seems very likely as I didn't think that he would just let his daughter die like that even if she wasn't his 'real' daughter. But he must know that she is! As it's really confusing!! Spoilers please!!!!!!