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The duke whom fell in love with Marie. He was born to his father's foreign mistress Liu-Liu, while the official duchess never bore a child; with the untimely demise of his father, along with Kyros's own accomplishments on the battlefield, he became the next duke without any backlash. He's loathed noble women who only ever wanted to marry him for status, which is why he was so drawn to Marie; she didn't care about status and had a caring heart. Even though he repeatedly explains to Marie that he got their names mixed up, Kyros is left utterly frustrated with her being unable to accept that she is NOT a replacement for her sister. This comedically leads to him showering her in unnecessary gifts and learning Marie has no clue as to how to choose what she likes on her own.
Kyros ultimately decides to bring the abuse Marie has suffered to the public eye, believing it will also help save her homeland from her father's incompetence.[source]https://en.wikipedia.org/wiki/Betrothed_to_My_Sister's_Ex#Characters[/source]