This arc was a mess. It really feels like Nagi only gives a shit about Aine because the plot demands it. He doesn't seem to like her at all, and was uncomfortable being around her. He actually seemed indifferent to her plight and the guild closing down right until the nobles decided they needed to send mercenaries after her. This is after they already braindrained her into a child and closed the guild. I can sort of understand taking the guild's assets as compensation for failing the sword quest, and implying the nobles guild was handling their debts. Why the memory theft though? That's apparently not normal. Why hold onto the crystal? Even an incompetent writer would hint that Aine or her grandfather knew a secret of the nobles, and all of this was in order to obtain that and then silence her but we get nothing.
It's even more forced than the slaves because she's not a beastman/demon living in hiding who Nagi can relate to, just a young woman that one noble really doesn't like for reasons. Is he he going to pick up every unmarried woman he runs into in each town?