Considered by most the overarching antagonist of the series, due to the apparent amount of pies his fingers are in. In Bakery Girl, he would be behind the Three Goddesses Project, declaring himself their Father. In Girls' Frontline, his actions would set forth the long-term plot of the game, to degrees both players and those within the story are still unsure of, even sparking uncertainty over his true identity for much of its story.

By Chapter 13 of Girls' Frontline, it is apparent that he is the leader of Paradeus. From Chapter 13 onward, it becomes clear that William is the brother of the original Lunasia, and son of Rudolf von Oberstein. Though initially framed as a pseudonym stolen from a now-deceased scientist, his full name would be revealed to be William von Oberstein.