To anyone who’s read the novel can I ask what dose the Duke need her for ?!

Spoiler but kind of not because it's within the first 12 chapters of the manhwa
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He was cursed and doesn't have a heart, so he's been using magic to keep his body alive but there's only a year until magic won't be enough and he'll die. When he saw her at a ball for the first time he felt his non-existent heart beat and he figured that she might know a way to break his curse and get his heart back. He's hoping she'll be able to break said curse and get his heart back.

As for if she succeeds I don't know. Haven't read the novel.