Count Drusus Beausoleil is a resident of Paris and a Professor of Philosophy and the Ars Mechanica at a university there, probably the Paris Institute of the Extraordinary, where he is/was a colleague of the currently-absent Professor Dio Zardeliv; he is occupying Zardeliv's office and keeping his clocks wound.
Or at least, this is the story told by the man upon first meeting Agatha in a Parisian pâtisserie. The whole truth turns out to be a bit more complicated. For one, the guy doing the telling is one of a collection of identical clank puppet-duplicates via which Beausoleil remotely interacts with the world; this first copy remains intact just long enough to distract Agatha's companions and allow another to kidnap her. She is taken to meet Beausoleil's nominal boss Du Quay, a would-be usurper of the Master of Paris. As he shows Agatha around the usurper's secret lair, Beausoleil states that he's unable to visit the location "in the flesh", but the reason for this remains unclear.
Before long, another layer of truth is peeled back: Drusus is in fact working for the Master in some official capacity, and dragged Agatha into contact with Du Quay in a covert effort to both thwart the man's plans and assess her abilities. This works quite spectacularly, but with the unfortunate side-effect that Agatha and a stolen piece of Castle Heterodyne's intellect threaten to take control of all of Paris, forcing the Master to get personally involved in the situation. Beausoleil is then assigned to be Agatha's official "Voice of Reason" for the three days which the Master allows her to remain in the city.
Following Agatha's adventures in the Paris underworld and the freeing of the now-monstrous Storm King from his temporal prison, Beausoleil is also revealed to be genuinely in league with some other as-yet-unidentified master as part of a plot to subvert the Master of Paris's rule; Beausoleil was tempted into this alliance by master #2's sharing with him some of the discoveries of the legendary Van Rijn, allowing the professor to greatly improve his clank-bodies. Before the enraged Master tears the relevant clank-body apart, it/he reveals he has wormed his way into indirect control of much of Paris's mechanical infrastructure.
Possibly Relevant Information Edit
"Beausoleil" translates to English as "Beautiful sun".
"Drusus" was a name in common use during the days of the Roman Empire. There was an entire clan that used the name "Drusus." One potential candidate for the basis of this character can be found in one Drusus Julius Caesar.