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"; see below for at least a partial explanation.
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 revealed to really be 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 copy apart, it/he reveals he has wormed his way into indirect control of much of Paris's mechanical infrastructure.
The ensuing battle against the Storm King ends in victory , but leaves Voltaire lying alone on a darkened Paris street, badly injured, possibly to the point of death, as Tweedle noisily celebrates his literal claiming of the Storm King's crown. Beausoleil pops up with a new body and stabs the Master in the back with an elaborate sword emblazoned with a Heterodyne trilobite insignia. It is revealed that, along with its primary purpose of definitively killing the Master, this weapon also completely cuts off its victim from the city network and sets up Agatha to be framed for its use. Beausoleil gloats about this whole situation, only for every alarm in the city to begin blaring, which is an announcement that the newly ascendant Spark/Mistress of Paris Colette Voltaire has learned of her father's death and who is responsible. She appears before the Master-killing body and with an enraged snap of her fingers, destroys all of his other incarnations within range of her power. In true Sparkish fashion, she employs every last method now at her vast command, and Beausoleil experiences every last bit of the attached agony. He nevertheless survives this, and declares that his actual physical body is not even in Paris. Colette comments that while he will survive for now, he will also have to deal with his presumably highly-displeased master(s), an assessment he nervously agrees with. She destroys the Master-killing body, banishing Beausoleil from the city.
Possibly Relevant Information
"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.