You can meet Arthur when we are sent into the book by that witch. He's a knight or something for the King, i believe you can only get him in two ways, i'm not sure but either: Chris needs to be alive and a part of your group OR had to have hooked up with your boss Morgan.
I noticed that in one of my plays, in which I DIDN'T have Chris because he died (i hadn't hooked up Morgan either); I ended up NOT getting Arthur but, you do meet a Servant girl instead.