The as-yet unnamed ruler of the Argurons, a technically-secret civilization living deep beneath Paris. When by Agatha and Co. that much of his court has been infested with Revenants, he proclaims that he was already well aware of this fact, and that he serves a "glorious Queen", presumably referring to some version of The Other. Whether he himself is a revenant has not been made entirely clear; either way, he displays a great deal of hammy enthusiasm for the idea.
Following a failed attempt to capture Agatha and her companions, he becomes the target of a successful kidnapping plot by Boris, Jenka and his own daughter Larana, and is taken as a unconscious prisoner to the nearby Immortal Library of the Grand Architect.
Unfortunately, while Agatha and her companions are searching through the Library's sealed Corbettite Vaults, he is nearly able to of the Library with the help of Slaver Wasps as supplied by his Geisterdame ally Madame Velix. However, this attempt is thwarted, and he is officially overthrown by Larana.
It is then revealed that he was (and still is) a professor of color theory at Transylvania Polygnostic University who "went rogue" twenty years ago while on a research trip, and married Larana's late mother. (It's an old, old story.) He is to be taken back to TPU to face the Board of Regents in regards to certain irregularities in his grant expenditures, a fate which perhaps even he does not deserve.
Outside Information Edit
The King looks and sounds an awful lot like Cheyenne Wright, or at least how the latter is in various GG bonus comics.
Because his daughter's name is Larana Chroma, it seems reasonable to assume the King's own surname is also Chroma. Since Cheyenne is the colorist for Girl Genius, this would seem to be something of an inside joke.