It says she is of one piece. I'd say there is a lack of conclusive information, and hence anything ventured at this point is wild speculation. I had a half formed idea that it may be related to age. Folly is young, and thus the energies she's exposed to haven't damaged as thoroughly as they have Sycorax or Efferus.Precisely, let the wild speculation fly!
I'd guess it has to do with her crystals & her Master's collection. I'll bet that the Cavendish lost a great deal of whatever held her sanity together. When Folly's Master (sorry can't remember his name currently & using my phone) last saw Cavendish, she must have been so far gone that he assumed she would certainly die, she might have also been aboard a ship crashing to the surface.That's fair I suppose. I was jumping to the assumption that being "Of One Piece" was the more rare state, rather than Cavendish being unusually broken. Which I suppose was based on the assumption that The Enemy is more broadly familiar with humans, if it's truly puppeteering Cavendish rather than just nudging her this way or that; whether it's the one that came up with the elaborate infiltration scheme or if that came from a human mind.
The Enemy came to her in that state & was able to fill her mental gap with its own will. The Enemy just assumed it could control Folly because she is likewise an Ethrealist like Cavendish.