I read it as psychological horror. A young man trapped in an extremely long period of lucid dreaming and the people around him unintentionally feeding those dreams with what they were reading to him which was twisted into battles between good and evil, sacrifice and constantly trying to achieve redemption. I think Dean was created so he wasn't alone and as a ferocious protector of 'Sam' who would do anything to save him. I do feel it as horror though because he has now awakened to find his personality literally torn in two and has to face the loss of not only what he believes to be his own identity but a gaping loss where his brother should be. Remarkably well done.