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So the novel's finished with 583 chapters (including the epilogues and IF stories), and there author decided to end the novel without giving Lizzy a canonical love interest. The final chapter of the Extra stories have her rejecting the protagonist of the sequel due to her being in a relationship already, but the identity of her spouse isn't revealed. (〜￣△￣)〜
As I said, there are five IF stories where she ends up with Robert (second prince), Christopher (adopted brother), Lilia (female protagonist), Edward (first prince), or Isaac (best friend). So there's no way to tell who Lizzy may end up with in the manga, if anyone. The manga-ka will either pick their favorite, or end the manga without Liz ending up with anyone. ╮(￣▽￣)╭
For reference, I like Lizzy's fake fiancé, Richard (the foreign prince) and Martin (Edward's bodyguard). They're definitely not endgame, though. Martin's straight-up not interested in Lizzy, and Lizzy and Richard love their siblings too much for either of them to marry into a different country. (≧∀≦)