Grandmother

Isabella and Krone were bad, but they changed and they have tragic backgrounds especially Isabella.

Grandmother Sarah on the other hand... is worse

Unless she lets her hair down to reveal both a beautiful visage and tragic backstory, I don't think I can forgive this monster.

Miss. I must ask you this from the bottom of my heart. I sincerely hope you will comply with my request....

Please die.

Is she just another pawn in the system or does she have a bigger role with the demons 

She is mysterious. I wonder if she will become an important character later.