Imaginary friend

An imaginary friend is a child's companion with complex personalities and interactions with the child, although the child realises that the friend is a pretence. Most children discard these figments by adolescence.

Adults' imaginary friends
Many young children have an imaginary friend. Christopher Robin had Binker. A little girl who wrote to me had a little purple man. And the girl with the little purple man actually saw him. She seemed to hallucinate him. He appeared with a little tinkling bell. And, he was very, very real to her, although in a sense she knew he wasn’t real. I suspect that something like that is going on with people who claim to have heard God or seen God or hear the voice of God.

Many adults, however, have a different type of imaginary friend that they believe to be real. This friend is normally invested with supernatural powers, often to the level of omnipotence. This friend can have varying degrees of influence on the life of the adult. Most commonly the moral code of the person is dictated by the friend, either through fear of retribution or respect for alleged past deeds of the friend, or of course a combination of the two. Some take the amount of control the friend has in their life to the extreme, allowing it to control the whole of their life and even usurp their reason.

Diagnosis
The difference from schizophrenia appears to be getting away with it. Also, thorazine doesn't do much for it.