Who is Jakob Gradus (a.k.a. Jack Degree, Jacques de Grey, James de from Pale Fire and what is their importance?

Gradus is a member of an extremist group called the Shadows. The Shadows are determined to locate and kill King Charles. Strangely enough, this part of Gradus' participation in the plot is fantasy spun by King Charles. Gradus is actually, Jack Grey, a recent escapee from a mental institution.