William is the protagonist of the story and the anti-hero.

He is a Math teacher at university and a very smart person that shows the others the face of an angel, but hides a demon inside (though, he hates to be epithetated like that).

His brothers and him made a path to reach the goal to free humanity from evilness, and they are ready to eveything to do that. So William is split between the love for his family and his comrades and the hate for the evilness. He also has a rivality wth Sherlock Holmes and he tries to use him to find crimes to punish, but he also thinks that Holmes is an incredible interesting person, because the two of them have marked intelligence but different way to see Justice.

William is ready to use death to make justice prevail, but Holmes doesn't want to sacrifice life, he wants to protect it.

And the more the story grows the more William has to face the fact that his actions have consequences on his mind and personality, because the darkness that he wants to destroy is even inside of him.