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I only read spoilers not the novel itself, and unfortunately for all of us, NO. This will not be a polyamorous relationship, at least not if the manhwa follows the novel. Ian and Louise will marry but Simon will travel the world. He will be single until the end. :(
As for Dean and Cleo, don't worry, they'll end up together.
Finally, Stella will regret her actions and becomes all right, she ends up with professor Hill (the grey haired botany teacher).
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Yep, the young grey haired teacher. He was also a poor noble who got sponsored by that bitch woman (forgot her name) and he regretede it a lot because he felt like he didn't get a chance to learn anything outside of botany because of her expectations and because his path in laife was already decided for him. So in a way. he was the one who understood Stella the most and the kind of pressure she was under. That's why they end up together.

If anybody has spoilers can you please tell me if this is and what a polyamorous relationship cuz likeeee ahemmmm