Lots of children have Imaginary Friends.

A boy at my first boarding school invented an Imaginary Brother, and got humiliated when he was found out.

But I'm the only person I know who had an Imaginary Enemy.  Fat with black hair.  Always walking up from behind and bullying.  Of course, I never told anyone about him.

You know, I could have sworn I saw him today, after all these years, following me up the road to the doctors'