Who is Sir Thursday. from Sir Thursday and what is their importance?

Sir Thursday. from Sir Thursday is one of the Morrow Days. He is the worst of them as he is trying to keep Arthur from the Key and teh Will. Though he would like to completely destroy Arthur, his code will not allow him to do so to someone serving in his group (Architect's Army). When his plan to trap and destroy Arthur fails, Arthur obtains the Key and the Will, then takes over as the leader of the Architect's Army, thus demoting Thursday.