Some people say alters are demons, other people say it's different personalities that are artificially created by splitting the core personality and programmed for different purposes. They are separated in brain somehow and demons only guard them. I saw it described like the brain is cabinet with drawers, in each drawer you have one alter, and each drawer is protected by lock that is demon. Fiona Barnett described her alters as girls that looked like her in the time when the alter was created, and that would support the theory of splitting of the core personality, I suppose.