"It iz a pleasure to watch hyu work!"
"...I can honestly say I've never heard that before."
Herr Veilchen is an assassin/sneaky guy employed by Anevka Sturmvoraus in her bid to take over Sturmhalten. Appearing early in Volume VI, he is tasked with accompanying the Rescue Party (consisting of Lars, Zeetha, Krosp, and da Boyz) through the sewers and into Sturmhalten Castle, winning Maxim's adoration while so doing.
When the Rescue Party becomes stranded in an oubliette, he escapes on his own using a grapnel gun (which only serves to enhance ) and after dealing with a sewer monster or three continues on with his real mission: to turn off the Lightning Moat and lower the drawbridge so that Anevka's minions can storm the castle. This doesn't turn out exactly as planned, but it's not his fault.
Inside the castle he meets with Tarvek, who gives Veilchen the key to the controls as planned, although the new arrival's stealthiness is somewhat impaired by the lingering after-effects of his trip through the sewers. The chaos that follows Tarvek's betrayal of his sister and the Baron's invasion of the town leaves his fate in question until he is seen in a flashback panel indicating that he was Violetta's the Great Hospital at Mechanicsburg. His arm is in a cast at that moment, indicating perhaps his departure from Sturmhalten was somewhat eventful.
Although nothing more about his precise position in Sturmhalten society is known, ever since Violetta appeared late in Volume VIII with a similar costume, allegiance, general mission, bag of tricks, and even name, (not to mention the whole "contact" thing) it's pretty much a given that he is, like herself, a Smoke Knight. If so, it is possible he is also a relative of the royal family rather than a commoner, and another of the very few people native to Sturmhalten who are not slaver-wasp-infected.
Possibly relevant outside information
"Veilchen", pronounced filechen ("ch" is like the Scottish "loch") is the German word for the flower violet (which matches the color of his cloak), but it is also a slang word in that language for "black eye" (blaues Auge, literally „blue eye")
The fact that "Violetta" is the Italian word for the flower violet is another indication of the two characters sharing a profession. With the Smoke Knights' wearing of violet, it might be common to name them after the flower in different languages.
On the other hand, his name could just be a pun -- he does have a veiled chin.