The Ultimate Goal
In the end, William will be someone who sees the world as a set of stories and looks for a way to insert himself. Morality is not important so much as the journey. The spiritual journey. The exception would be his story, and finds where to insert himself. Like Ashan, he will feel mages are blasphemous to the soul, and will seek to champion stories where they are unmarked in exchange for some kind of reward, even if he is aiding them towards their goals as a spiritual guide. He will intentionally limit himself, to make the story more interesting - wielding a weapon he is not skilled at, or refusing to use certain abilities or tactics that give him an edge over the characters he interacts with. Some form of Illusion where he changes what he looks like to blend in with the world at large, so his fame does not precede him - perhaps with alter egos. This is the character I want William to become.
The marks that ultimately make most sense for William are Cassion's, Ashan's, Jesine's, and possibly Aeva's. Will will see being Cassion's champion as something that robs him of the capability to truly weave his stories without limitation, and will prefer to be Ashan or Aeva's champion, unless Aeva is foregone for another.