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Baron Klaus Wulfenbach was once a companion of the Heterodyne Boys, who were the last of a long line of Mad Scientists (and occasional dictators) in the continent of Europa. Wulfenbach and the Heterodynes faced and ultimately defeated a powerful adversary known as The Other, after which Wulfenbach disappeared for reasons that have not yet been made clear. Klaus eventually returned to Europa to find the Heterodynes gone and himself in the middle of a war between the multitude of royal families that control the continent. Fed up with the petty squabbling of individual interest, Wulfenbach used the knowledge and power that came from his spark (a gift of supreme genius granted to certain individuals in the Girl Genius world that allows amazing developments and creations in science and mechanics) to slowly conquer the individual principalities that make up Europa and unite them under his banner.
Baron Wulfenbach rules his empire with an iron fist from Castle Wulfenbach, a massive airship that is effectively its own city, and which is kept afloat at all times.
Wulfenbach has a son, Gilgamesh Wulfenbach (most often called simply 'Gil') who Klaus is continually testing in order to take the measure of the man who will very likely inherit the empire. Gil was kept unaware of his parentage for many years, living with a collection of other Europa royal children who are kept aboard Castle Wulfenbach as insurance against their parents taking action towards Klaus. Currently, Gil finds himself in love with Agatha Heterodyne, a development that Klaus fears because of Agatha's parentage and potential. In his mind, Agatha could undo all the peace and order that Klaus has slowly achieved.
Before the defeat of The Other, Klaus had a relationship with Agatha's mother, who eventually left Klaus to marry one of the Heterodyne Boys. Agatha's mother, Lucrezia Mongfish Heterodyne, turns out to be "The Other", and Klaus must spend much of the time between his disappearance and the beginning of the Girl Genius series cleaning up her messes.