Martian Boyhunter

At first, it seems Martian Boyhunter is just an imaginary friend J'onn J'onzz made up so he wouldn't feel left out with the other Justice League members having their own sidekicks.<br><br>Once the other heroes start beating up J'onn in retribution for killing their sidekicks, Martian Boyhunter turns out to be real and laughs at getting his mentor in trouble.

Example of:
Not-So-Imaginary Friend