Who is Jaffe/The Jaff from The Great and Secret Show: The First Book of the Art and what is their importance?

Randolph Jaffe is a man of little ambition and less in the way of prospects until he discovers great secrets in letters that find their way into the dead letter room in Omaha, Nebraska. From these letters, he understands great secrets written in them, and comes to understand that the world everyone knows is false, and that there is a deeper world behind it all. He murders his boss, Homer, who becomes suspicious of his activities. From there Jaffe embarks on a journey across America that leads him to the ancient Shaman Kissoon, a man who promises to teach him the secrets of the Quiddity, but Jaffe immediately dislikes the man and sets about to entering the dream sea in his own way. He employs an evolutionary biologist named Fletcher, who uncovers the secrets of a fluid he calls the Nuncio, that automatically evolves whatever living tissue it comes into contact with. Jaffe manages to capture a vial of the substance, and turns into the Jaff, a creature of darkness who seeks nothing but power and the ability to enter the Quiddity. The two battle each other and wind up in Palomo Grove, where they inspire several children to be born. The Jaff fathers Tommy-Ray and Jo-Beth, and attempts to use them in his battle against Fletcher. Eventually, he succeeds in opening a path to the Quiddity, only to realize too late that he has invited a great disaster upon the world by doing so. In the end battle to protect the earth, he kills Kissoon and stays with his son, Tommy-Ray, as the atomic bomb goes off, destroying the rift.