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

Fletcher is an evolutionary biologist. He was originally employed by Jaffe in order to uncover the secrets of what makes life evolve. Over time, he grows suspicious of Jaffe's motives and attempts to destroy his discovery, which is called the Nuncio. Unfortunately, the Nuncio has a will of its own and shatters its vial to reach him. Once doing so, it transforms him into a godlike creature.