SpoilerWilliam is the second and youngest child of the Duke and Lady Moriarty.
He has never accepted the two orphans his family had welcomed because "nobility obliges". He's being cruel towards the boy who would take his name, not hesitating to pretend to be a victim in order that the orphan would be punished.
Alongside his mother, William James Moriarty was killed by the boy who would take his name, as well as Louis and his older brother Albert.