The Green man is a "Deus Ex Machina" that appears in the novel Insomnia. Not much is known about the Green man. He appears to Lois and tells her to give Ralph her earrings that she previously had stolen by Atropos. In doing so, Ralph was able to surprise the Crimson King by stabbing him with Lois's earrings which broke the Catfish illusion the Crimson King had on. This gave Ralph the chance to set off his booby-trapped arm to drive away the Crimson King.
The Green man is described as being "good" and has a green aura. Since he helped Ralph fight the Crimson King, he can be considered to be on the purpose.
Not much is known of the Green Man since he doesn't appear in any other Stephen King novels, but he could be the energy of Maturin because of his green aura. He can also be a manifestation of Gan.