"Grovels nicely, doesn't he?"
- --Castle Heterodyne
Herr Wilhelm Diamant is in charge of supplying Castle Heterodyne with both new prisoners[1] and their supplies. He reports to Baron Klaus Wulfenbach on the activities of his job. His official vehicle is a curiously-constructed prisoner transport van, badged with both the Heterodyne trilobite and the Wulfenbach winged tower on a field of per chevron inverted gules and azure.[2]
When first confronting the Castle and Carson von Mekkhan about Agatha and Carson using the Throne of Faustus Heterodyne again, he scoffs at Agatha's authenticity[3]. However, he is quickly converted[4] when Agatha talks the Castle down from vivisecting Diamant[5] for his insolence.
He later shows Agatha through some of the Castle's intricate passages up to the outer walls before forging her papers condemning her to Castle imprisonment and transporting her to the front doors.
After the Doom Bell rings, he shows up with several other city elders and Vanamonde von Mekkhan in formally swearing fealty to Agatha. He is presumably trapped in Mechanicsburg when Klaus activates his Take-Five Bomb.
He evidently was also the one who sent Sanaa into the Castle, as she used "Wilhelm" as an alias, directly following up on his suggestion she not reveal her real last name.
Possibly relevant outside information[]
"Diamant" means "diamond" (just in case you didn't guess).