Whatever works for the quest.Agatha is indeed a big ghost, but she is just a ghost and not even conscious of what she is doing, more of an impression than a person. Her stealing a part of Faerie feels a bit like a rumba annexing territory, she should not really have the capacity to stand up to intelligent opposition.
This is what Dresden Files Accelerated has to say, specifically referencing Agatha:
Demesnes
For the most part, denizens of the Nevernever have claimed territory as their own, with little shifting in the borders throughout the millennia despite
the efforts of Summer and Winter to expand their own lands and contract the other's. The areas claimed also change to fit the needs of their claimants or, perhaps, the other way around.
But sometimes the supernatural beings without land of their own have sufficient will to create their own environment, a comparatively small bubble of
reality within the Nevernever. This bubble is called a demesne, and thousands of them are scattered across the supernatural world, burrowed in niches here and there where the fabric of existence could stretch to accommodate them.
One such place is a slice of turn-of-the-century Chicago that an old, tormented, demented spirit named Agatha Hagglethorn created on the edge
of the Nevernever that connects with modern-day Chicago. Despite events allowing some peace for Agatha's ghost, the demesne remains. Even in the
Nevernever, creation matters.
Another demesne, also mirroring historical Chicago, has been built in the guise of a CPD police station and the surrounding city blocks, including a
train station. Those who are responsible for this demesne ensure its security, as it represents a Waypoint between the mortal realm and what follows a mortal's demise. I may add few details to what can be gleaned from Wizard
Dresden's notes: a guardian angel protects the entrance, a late Black Cat detective named Colin Murphy is its spiritual overseer, and a second spirit, Ron Carmichael, is one of his aides.
I assume this Black Cat department demesne is, like Agatha's demesne, now a permanent fixture in the Nevernever. Anything that occurred around Wizard Dresden's death and return is clouded, even from me, and thus we have only Harry's recollections to use.
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Source is Dresden Files Accelerated RPG p67