Perhaps Niciel's mind would be a peaceful paradise, a sprawling field of different-colored flowers, extending beyond the horizon, with a bright sun above, and just a single house in the middle of it all, complete with furniture and different rooms. I figure the house would be the place that Niciel was living when she was still a mortal, before she became Niciel. Inside the house would also be a single human female, which would be the mortal who had become Niciel. 

The mortal in Niciel's mind would be rather empty, though, having a perpetual blank expression and performing the mundane tasks of a mortal housewife. Attempting to communicate with her would get you a stare, possibly a few words too, before she returns to her task.

I would write out other details, but this will suffice until a trip into Niciel's mind is actually made.