Captain Abelard is the skipper of the Stockholm airship which carried Duke Strinbeck, Baron Oublenmach, and Zola to Mechanicsburg. Little is known about him, but he's not happy with his passengers, his mission, or his ship (a fine vessel, but a bit on the pink side for his tastes). He is probably not working for the Knights of Jove on any permanent basis. He certainly isn't fond of the ones he's met.
He is not fond of violations of Europan airship etiquette, and is alerted by the behaviour of his experienced helmsman, Kraddok. Understandably, he does not like people waving guns around on his dirigible. He especially does not like people threatening his men with guns. Getting talked down to by both his clients, is just too much for him one day, so when Duke Strinbeck over indulges in a fit of bad temper just as the ship is fleeing the Torchmen, and is in desperate need of ditching useless unneeded weight, Strinbeck is, to put it politely, unceremoniously defenestrated. The Captain, now running his own ship again, wants to be anywhere but Mechanicsburg. Unfortunately for him, when last seen, he and his ship have Castle Wulfenbach.
In the Girl Genius Complete List of Absolutely Everybody! he is identified as Un-named Airship Captain. Agatha H. and the Voice of the Castle names him Captain Abelard. He's not to be confused with the named-but-never-seen Captain Patel, skipper of Castle Wulfenbach.