She is ambitious…. You will eventually read in future chapters how she will start making bad decisions, be manipulated, and eventually turn evil herself She allowed herself to be used by the cult voluntarily… so she can have what the upper nobility has, what debora has.
Probably because 1. She can't pull the sword so she can't take it with her. 2. Who will she entrust the protection to? The white beast? Even if he was protecting it, he can't be there 24/7 as FL still needs him sometimes.
At this point, the second prince is surprisingly a better choice than his obsessive brother (〜 ̄△ ̄)〜
HAHAHAHA real