Dean in many ways is the guy that basically makes things happen for Sal and throughout the novel. Even though Sal knows that sometimes Dean is dangerous and that he isn't always very considerate (or really considerate at all) he is still drawn to him and his crazy energy.
Dean is always looking for something to get into and he is looking for something to "dig," and will go to great lengths to find it. There are also hints in his character that point back to his rather dark past and the desperate circumstances he grew up in.
By the end of the novel, he serves as sort of the warning of what can happen when you pursue things without any consideration for the circumstances or the future as he ends up penniless and really crazy wandering the streets.