Saturnus Heterodyne was the father of Barry Heterodyne and William Heterodyne, and paternal grandfather of Agatha Heterodyne. His name is first mentioned in the comic The novel Agatha H. and the Voice of the Castle changes his name to "Saturn".
He built the sapient railway train The Beast, which is described by one of its less-than-thrilled recipients as being a good reflection of its creator, capricious, ravenous and proud. As ruler of Mechanicsburg, it was his policy to in order to get a better look at the weapons and equipment of attacking armies. Additionally, during the "Year with Three Winters" he to the "old" town hall and kept the conflagration burning for five months.
According to the Secret Blueprints and the above-mentioned novel he genuinely fell in love with his would-be wife, the noble and brave Teodora Vodenicharova, and forced her to marry him on threat of harm to her family (once again showing himself to be a great believer in his own family tradition.) She was the one responsible for raising William and Barry to become the heroes that they were, and when Saturnus indicated his intention to kill them and start over from scratch in the whole heir-production department, Teodora fatally poisoned him, and was killed in turn by Castle Heterodyne.