It's the obssessive tendency of who they are. (Sorry, can't remember the name of their powers)
They find peace with her near them but other then her bother they don't really know why.
and those who do not know why, resort to obsession, though I think aseras knows why but well you know, in the very beginning he really hated the empire already for some reason, so it got nothing to do with his powers
and dahlia's powers is affected by how much he cares for the said person, and knowing his the one who tried to hurt the duchess, her powers would not be very effective on him
Well as far as i remember :
1. It's tendencies of transcendans to be obsessed with someone they like that's why they also obsessed with Adrisha in the original game also the reason why her power is being hidden in the first place
2. Aceras want to be the pope which basically like the king / highest authority of the holy empire which using his power to the point of no return that's why he need Dahlia in the first place
3. The emperor and Hikan hide her power the best he could it's no way they will hand Dahlia just like that and Aseras doesn't know that her power link to her relationship to the said person yet that's why he want to truly lock her up for him and himself alone that's why he want no one to have access to her thus the plan to destroy the Frederick Empire.
Why can't the pope ask her to purify him timely? I don't think dahlia would refuse...
I forgot some parts is he trying to keep her for himself or sum?