The main character's alter-ego in the Chuck Palahniuk novel (and movie) "Fight Club". A representation of the narrator's id, who is mentally and emotionally capable of doing things that the narrator cannot, but desperately wants to.
What people dont understand is that Tyler Durden and Jack are the same thing. They BOTH live and thrive off each other. Until Jack came to the realization that he needed to live without him. He was his best friend and confidant and his soul tie. It was so much more than an alter ego.
The way we persceve our selves is the way we peraceve our best friends. We are our friends and they are us. With out them we are nothing.