[ Spoiler ]
William is a pretty odd case, as far as the normal street cast goes. Once a law-school graduate and fledling lawyer his life took a turn for the ...interesting when his till then dormant magical talent awakened. At this point of his life he was becoming increasingly fed up with the legal system and standard law procedures, especially when a long term client of his was sentenced to death despite valid doubts on the accusations simply because of a bureaucratic oversight.
As a "late waker" his awakening proved to be particularly violent when months of pent up frustration forced their way out during a trial in an outburst of malevolent raw mana. Causing considerable devastation in the courtroom and injuring several of the officials he also for the first time made contact with the entity he later came to know as "Death" and which later revealed herself to him as his Mentorspirit. With her help he managed to flee the scene when he suddenly found himself on the wrong side of the law as an illegal magician charged with premeditated magical assault and attempted murder.
Without access to any of his assets (he was deep in debt anyway from tuition fees) and suddenly becoming a wanted criminal he was forced to disappear from the screen, and without the necessary contacts nor money for a new identity he ventured into the one place he knew the Star would not come looking for him. The Redmond Barrens.
Survival proved to be tough, even with the help of his Mentor which rarely strayed from his side during that time. (She claimed to have business here.) But somehow he found himself to be the patron of a small gang of street kids who seemed to be drawn to him. Life got a bit better then for a while. He would look after the kids, tend to their wounds or diseases, and in turn they began to teach him the ways of the street. This was cut short by the events of Red October and especially Halloween, when a squad of feral 162s ghouls raided the hideout William had set up for him and the kids while he was out looking for some sweets for the kids.
After the events of All Hallow's Eve Will's actions all carry a certain amount of fatality. He deeply blames himself and his passiveness for the deaths of his charges. The relation with his mentor will probably suffer from this too. On the one hand he understands her basic principles about life and death, the inevitability of which and the meaning that underlies it. But at the same time he deeply suspects that she might have purposefully steered him into this position to exactly "make her point clear", and he can't yet forgive her for that.
It was already stated that Will is a somewhat odd case on the streets. This is emphasized by the fact that he is still wearing his old business suit and tie, as torn and rugged as they might be by now. Also he seems to be constantly surrounded by one or more of his watcher spirits which take the form of a small school of bright, little goldfish seemingly using the air as water.
this thing is meant as an addendum for the second draft short story introduction of Will. Even though it is a bit short yet, it makes more sense than the very first version, if only because it is more humble.
i also found a picture which is near to what he looks like in my head.
http://stan522.deviantart.com/art/Man-in-suit-16601854
one of things that fascinate me about William is the "anachronism" that he is on the streets.