You possess a second mind within your head that carries different values and goals from you. They may not carry agree to whatever you want to do and differ from you majorly.
If you possess Mild Hallucination, they are just a voice in your head that you can control when to swap. If you possess Hallucinations you and them can sometimes swap between each other, you have to fight for control. If you possess Major hallucination you are the voice in their mind and they control when to swap.