You possess a second mind within your head that carries the same values and goals as you. They are usually agreeable to whatever your goal is. But usually how this is achieved can differ from how you want it to. Additionally, this other consciousness is usually fine with not being in control. But they will fight for control if you do something they don't agree with.
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.