You afraid that she's actually SUPER smart & Savvy, and is taking advantage of Eru's first love to accomplish what she was sent for? Maybe. Though, I suspect that even if that weren't the case, her biological old man is unlikely to be pleased at the prospect of his "chess piece" getting "ideas of her own" and "going out and marrying the foot soldier of the count". On the other hand, Eru is probably an ideal choice because he's so close with Vel. If you can't marry the Monarch/Noble, then go for one of the generals. LOL.
Because you have this feeling that she IS a reincarnator like Vel, and is in fact his ex. ROFL. Well, maybe. I don't know. That said, I rather doubt that she'd still end up with Vel at this point.
As much as I care for Eru and want the best for him, he's definitely been fooled by this girl, she immediately figured his personality out and took advantage of that knowledge to the best of her ability, and her secretly being - my guess here - a master manipulator means her ability is very high.
This reminds me of The Mrs. in Tensei Kizoku, Kantei Skill de Nariagaru ~ Jakushou Ryouchi wo Uketsuida node, Yuushuu na Jinzai wo Fuyashiteitara, Saikyou Ryouchi ni Natteta ~ | As a Reincarnated Aristocrat, I'll Use My Appraisal Skill to Rise in the World who is a dangerously effective tactician when it comes to social chessmastering. LOL. She's DEFINITELY the final boss. Maybe, Karla will turn out to be one too?