More of man made phenomenon than a character. This is a case of Klaus having imprinted his own personality in his son's, Gilgamesh's, brain, through a similar but process then Lucrezia's. Klaus to accept his imprint during The Siege of Mechanicsburg when Klaus was threatening Agatha's life. Klaus threatened Agatha's life because he knew that Gil would die for her, and threatening Gil would be worse than useless. Gil did his best to hold out, but after .
The process was not shown on panel, however evidence of its manifestation occurred was when Gil tried to rescue Agatha from the top of Castle Heterodyne's gate tower. Franz had so that she could repair the switchgear attached to the lightning rods atop the tall tower, thereby allowing Castle Heterodyne to recharge itself by capturing power from the raging storm. Agatha was deeply suspicious of Gil the instant she saw him and fought him and his Hunter clanks off, nearly falling from the tower.
Agatha is hag-ridden as well, by her mother, Lucrezia Mongfish. Dimo refers to this kind of condition as a . The Klaus-in-Gil dybbuk is very much like the Lucrezia-in-Agatha dybbuk in that its control is sporadic and it is unable to make itself felt full time. The Klaus-in-Gil is so powerful that it has a strong influence on Klaus's minions such as Bangladesh DuPree. She treats Gil just like she does his father--with gleeful respect that has the appearance of disrespect. Dupree was later .
So far as we know at this time, a dybbuk is only able to infect and influence biological beings. Lucrezia simply took over Anevka Sturmvoraus's body rather than merely occupying a corner of her mind. There is also a dybbuk in Zola Malfeazium that is from Lucrezia. It remains unclear at this time whether a dybbuk could successfully infect the mind of a biological construct such as Punch or Judy.